Problem:    Vim9: no error for missing space after comma in dict.
Solution:   Check for white space. (closes #6672)

Problem:    Vim9: no specific error when parsing lambda fails.
Solution:   Also give syntax errors when not evaluating. (closes #7154)

Problem:    Vim9: "!" followed by space incorrectly used.
Solution:   Skip over trailing spaces. (closes #7131)

Problem:    Vim9: cannot insert a comment line in an expression.
Solution:   Skip comment lines at the script level. (closes #7111)

Problem:    Vim9: crash with unterminated dict. (Dhiraj Mishra)
Solution:   Return empty string instead of NULL. (closes #7084)

Problem:    Vim9: trinary operator condition is too permissive.
Solution:   Use tv_get_bool_chk().

Problem:    Vim9: operators && and || have a confusing result.
Solution:   Make the result a boolean.

Problem:    No falsy Coalescing operator.
Solution:   Add the "??" operator.  Fix mistake with function argument count.

Problem:    Vim9: crash with invalid list constant. (Dhiraj Mishra)
Solution:   Return FAIL when compiling the list fails. (closes #7066)

Problem:    Vim9: Some tests are still using :let.
Solution:   Change the last few declarations to use :var.