Problem:    No space allowed before comma in list.
Solution:   Legacy Vim script allows it. (closes #6577)

Problem:    Vim9: skipping over white space after list.
Solution:   Do not skip white space, a following [] would be misinterpreted.
            (closes #6552)  Fix a few side effects.

Problem:    Vim9: checking for Vim9 syntax is spread out.
Solution:   Use in_vim9script().

Problem:    Vim9: double quote can be a string or a comment.
Solution:   Only support comments starting with # to avoid confusion.

Problem:    Vim9: line continuation does not work in function arguments.
Solution:   Pass "evalarg" to get_func_tv().  Fix seeing double quoted string
            as comment.

Problem:    Vim9: no line break allowed after some operators.
Solution:   Skip a line break after the operator.  Add
            eval_may_get_next_line() to simplify checking for a line break.

Problem:    Vim9: leaking memory when lacking white space in dict.
Solution:   Clear the typval.

Problem:    Vim9: no line break allowed inside a dict.
Solution:   Handle line break inside a dict in Vim9 script.

Problem:    Vim9: script cannot use line continuation like in a :def function.
Solution:   Pass the getline function pointer to the eval() functions.  Use it
            for addition and multiplication operators.

Problem:    Many type casts are used for vim_strnsave().
Solution:   Make the length argument size_t instead of int. (Ken Takata,
            closes #5633)  Remove some type casts.