Problem:    Vim9: allowing both quoted and # comments is confusing.
Solution:   Only support # comments in Vim9 script.

Problem:    Vim9: invalid type error for function default value.
Solution:   Use right argument index. (closes #6458)

Problem:    Vim9: cannot use 'text'->func().
Solution:   Recognize string at start of command.

Problem:    Vim9: crash when passing number as dict key.
Solution:   Check key type to be string. (closes #6449)

Problem:    Vim9: crash when function calls itself.
Solution:   Add status UF_COMPILING. (closes #6441)

Problem:    assert_fails() checks the last error message.
Solution:   Check the first error, it is more relevant.  Fix all the tests
            that rely on the old behavior.

Problem:    Vim9: filter function recognized as command modifier, leading to a
            crash.
Solution:   Clear cmdmod after freeing items.  Do not recognize a command
            modifier followed by non-white space. (closes #6434)

Problem:    Vim9: not enough type checking in Vim9 script.
Solution:   Use same type checking as in a :def function.

Problem:    Vim9: Cannot split a line before ".member".
Solution:   Check for ".member" after line break.

Problem:    Vim9: builtin function method call only supports first argument.
Solution:   Shift arguments when needed. (closes #6305, closes #6419)