patch 8.2.1515: Vim9: can create s:var in legacy script but cannot unlet

Problem:    Vim9: can create s:var in legacy script but cannot unlet.
Solution:   Allow :unlet for legacy script var.

src/testdir/test_vim9_script.vim

@@ -608,6 +608,13 @@ def Test_unlet()
   assert_false(exists('g:somevar'))
   unlet! g:somevar
 
+  # also works for script-local variable in legacy Vim script
+  s:somevar = 'legacy'
+  assert_true(exists('s:somevar'))
+  unlet s:somevar
+  assert_false(exists('s:somevar'))
+  unlet! s:somevar
+
   call CheckScriptFailure([
         'vim9script',
         'let svar = 123',

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1515,
 /**/
     1514,
 /**/

src/vim9compile.c

@@ -258,6 +258,15 @@ lookup_arg(
     return FAIL;
 }
 
+/*
+ * Returnd TRUE if the script context is Vim9 script.
+ */
+    static int
+script_is_vim9()
+{
+    return SCRIPT_ITEM(current_sctx.sc_sid)->sn_version == SCRIPT_VERSION_VIM9;
+}
+
 /*
  * Lookup a variable in the current script.
  * If "vim9script" is TRUE the script must be Vim9 script.  Used for "var"
@@ -271,8 +280,7 @@ lookup_script(char_u *name, size_t len, int vim9script)
     hashtab_T	    *ht = &SCRIPT_VARS(current_sctx.sc_sid);
     dictitem_T	    *di;
 
-    if (vim9script && SCRIPT_ITEM(current_sctx.sc_sid)->sn_version
-							!= SCRIPT_VERSION_VIM9)
+    if (vim9script && !script_is_vim9())
 	return FAIL;
     cc = name[len];
     name[len] = NUL;
@@ -5234,6 +5242,9 @@ check_vim9_unlet(char_u *name)
 {
     if (name[1] != ':' || vim_strchr((char_u *)"gwtb", *name) == NULL)
     {
+	// "unlet s:var" is allowed in legacy script.
+	if (*name == 's' && !script_is_vim9())
+	    return OK;
 	semsg(_(e_cannot_unlet_str), name);
 	return FAIL;
     }