patch 9.0.1031: Vim9 class is not implemented yet

Problem:    Vim9 class is not implemented yet.
Solution:   Add very basic class support.

src/structs.h

@@ -1406,6 +1406,9 @@ typedef struct {
 typedef struct isn_S isn_T;	    // instruction
 typedef struct dfunc_S dfunc_T;	    // :def function
 
+typedef struct type_S type_T;
+typedef struct ufunc_S ufunc_T;
+
 typedef struct jobvar_S job_T;
 typedef struct readq_S readq_T;
 typedef struct writeq_S writeq_T;
@@ -1415,6 +1418,8 @@ typedef struct channel_S channel_T;
 typedef struct cctx_S cctx_T;
 typedef struct ectx_S ectx_T;
 typedef struct instr_S instr_T;
+typedef struct class_S class_T;
+typedef struct object_S object_T;
 
 typedef enum
 {
@@ -1434,16 +1439,18 @@ typedef enum
     VAR_JOB,		// "v_job" is used
     VAR_CHANNEL,	// "v_channel" is used
     VAR_INSTR,		// "v_instr" is used
+    VAR_CLASS,		// "v_class" is used
+    VAR_OBJECT,		// "v_object" is used
 } vartype_T;
 
 // A type specification.
-typedef struct type_S type_T;
 struct type_S {
     vartype_T	    tt_type;
     int8_T	    tt_argcount;    // for func, incl. vararg, -1 for unknown
     int8_T	    tt_min_argcount; // number of non-optional arguments
     char_u	    tt_flags;	    // TTFLAG_ values
     type_T	    *tt_member;	    // for list, dict, func return type
+				    // for class: class_T
     type_T	    **tt_args;	    // func argument types, allocated
 };
 
@@ -1452,6 +1459,38 @@ typedef struct {
     type_T	*type_decl;	    // declared type or equal to type_current
 } type2_T;
 
+/*
+ * Entry for an object member variable.
+ */
+typedef struct {
+    char_u	*om_name;  // allocated
+    type_T	*om_type;
+} objmember_T;
+
+// "class_T": used for v_class of typval of VAR_CLASS
+struct class_S
+{
+    char_u	*class_name;		// allocated
+    int		class_refcount;
+
+    int		class_obj_member_count;
+    objmember_T	*class_obj_members;	// allocated
+
+    int		class_obj_method_count;
+    ufunc_T	**class_obj_methods;	// allocated
+    ufunc_T	*class_new_func;	// new() function that was created
+
+    garray_T	class_type_list;	// used for type pointers
+    type_T	class_type;
+};
+
+// Used for v_object of typval of VAR_OBJECT.
+// The member variables follow in an array of typval_T.
+struct object_S {
+    class_T	*obj_class;  // class this object is created for
+    int		obj_refcount;
+};
+
 #define TTFLAG_VARARGS	0x01	    // func args ends with "..."
 #define TTFLAG_BOOL_OK	0x02	    // can be converted to bool
 #define TTFLAG_STATIC	0x04	    // one of the static types, e.g. t_any
@@ -1467,17 +1506,19 @@ typedef struct
     union
     {
 	varnumber_T	v_number;	// number value
-	float_T		v_float;	// floating number value
-	char_u		*v_string;	// string value (can be NULL!)
-	list_T		*v_list;	// list value (can be NULL!)
-	dict_T		*v_dict;	// dict value (can be NULL!)
+	float_T		v_float;	// floating point number value
+	char_u		*v_string;	// string value (can be NULL)
+	list_T		*v_list;	// list value (can be NULL)
+	dict_T		*v_dict;	// dict value (can be NULL)
 	partial_T	*v_partial;	// closure: function with args
 #ifdef FEAT_JOB_CHANNEL
-	job_T		*v_job;		// job value (can be NULL!)
-	channel_T	*v_channel;	// channel value (can be NULL!)
+	job_T		*v_job;		// job value (can be NULL)
+	channel_T	*v_channel;	// channel value (can be NULL)
 #endif
-	blob_T		*v_blob;	// blob value (can be NULL!)
+	blob_T		*v_blob;	// blob value (can be NULL)
 	instr_T		*v_instr;	// instructions to execute
+	class_T		*v_class;	// class value (can be NULL)
+	object_T	*v_object;	// object value (can be NULL)
     }		vval;
 } typval_T;
 
@@ -1663,7 +1704,7 @@ typedef enum {
  * Structure to hold info for a user function.
  * When adding a field check copy_lambda_to_global_func().
  */
-typedef struct
+struct ufunc_S
 {
     int		uf_varargs;	// variable nr of arguments (old style)
     int		uf_flags;	// FC_ flags
@@ -1671,6 +1712,9 @@ typedef struct
     int		uf_cleared;	// func_clear() was already called
     def_status_T uf_def_status; // UF_NOT_COMPILED, UF_TO_BE_COMPILED, etc.
     int		uf_dfunc_idx;	// only valid if uf_def_status is UF_COMPILED
+
+    class_T	*uf_class;	// for object method and constructor
+
     garray_T	uf_args;	// arguments, including optional arguments
     garray_T	uf_def_args;	// default argument expressions
     int		uf_args_visible; // normally uf_args.ga_len, less when
@@ -1731,7 +1775,7 @@ typedef struct
     char_u	uf_name[4];	// name of function (actual size equals name);
 				// can start with <SNR>123_ (<SNR> is K_SPECIAL
 				// KS_EXTRA KE_SNR)
-} ufunc_T;
+};
 
 // flags used in uf_flags
 #define FC_ABORT    0x01	// abort function on error
@@ -1750,6 +1794,9 @@ typedef struct
 				// copy_lambda_to_global_func()
 #define FC_LAMBDA   0x2000	// one line "return {expr}"
 
+#define FC_OBJECT   010000	// object method
+#define FC_NEW	    030000	// constructor (also an object method)
+
 #define MAX_FUNC_ARGS	20	// maximum number of function arguments
 #define VAR_SHORT_LEN	20	// short variable name length
 #define FIXVAR_CNT	12	// number of fixed variables