Problem: Vim9: list of list type can be wrong.
Solution: Use VAR_UNKNOWN for empty list. Recognize VAR_UNKNOWN when
looking for a common type. (closes#6979)
Problem: Vim9: list<any> is not accepted where list<number> is expected.
Solution: Add functions to allocate and free a type_T, use it in
ISN_CHECKTYPE. (closes#6959)
Problem: Vim9: result of && and || expression cannot be assigned to a bool
at the script level.
Solution: Add the VAR_BOOL_OK flag. Convert to bool when needed.
Problem: Vim9: white space checks are only done for a :def function.
Solution: Also do checks at the script level. Adjust the name of a few
error messages.
Problem: Vim9: list type at script level only uses first item.
Solution: Use all members, like in a compiled function. (closes#6712)
Also for dictionary.
