patch 8.2.2449: Vim9: flatten() always changes the list type

Problem:    Vim9: flatten() always changes the list type.
Solution:   Disallow using flatten() and add flattennew().

runtime/doc/eval.txt

@@ -2549,6 +2549,8 @@ finddir({name} [, {path} [, {count}]])
 findfile({name} [, {path} [, {count}]])
 				String	find file {name} in {path}
 flatten({list} [, {maxdepth}])	List	flatten {list} up to {maxdepth} levels
+flattennew({list} [, {maxdepth}])
+				List	flatten a copy of {list}
 float2nr({expr})		Number	convert Float {expr} to a Number
 floor({expr})			Float	round {expr} down
 fmod({expr1}, {expr2})		Float	remainder of {expr1} / {expr2}
@@ -4712,8 +4714,10 @@ flatten({list} [, {maxdepth}])					*flatten()*
 		Flatten {list} up to {maxdepth} levels.  Without {maxdepth}
 		the result is a |List| without nesting, as if {maxdepth} is
 		a very large number.
-		The {list} is changed in place, make a copy first if you do
+		The {list} is changed in place, use |flattennew()| if you do
 		not want that.
+		In Vim9 script flatten() cannot be used, you must always use
+		|flattennew()|.
 								*E900*
 		{maxdepth} means how deep in nested lists changes are made.
 		{list} is not modified when {maxdepth} is 0.
@@ -4727,6 +4731,10 @@ flatten({list} [, {maxdepth}])					*flatten()*
 			:echo flatten([1, [2, [3, 4]], 5], 1)
 <			[1, 2, [3, 4], 5]
 
+flattennew({list} [, {maxdepth}])			*flattennew()*
+		Like |flatten()| but first make a copy of {list}.
+
+
 float2nr({expr})					*float2nr()*
 		Convert {expr} to a Number by omitting the part after the
 		decimal point.

runtime/doc/usr_41.txt

@@ -665,6 +665,7 @@ List manipulation:					*list-functions*
 	count()			count number of times a value appears in a List
 	repeat()		repeat a List multiple times
 	flatten()		flatten a List
+	flattennew()		flatten a copy of a List
 
 Dictionary manipulation:				*dict-functions*
 	get()			get an entry without an error for a wrong key