Problem:    Vim9: argument to add() not checked for blob.
Solution:   Add the BLOBAPPEND instruction.

Problem:    Vim9: add() does not check type of argument.
Solution:   Inline the add() call. (closes #7160)

Problem:    Vim9: get error message when nothing is wrong.
Solution:   Check called_emsg instead of did_emsg. (closes #7143)

Problem:    Vim9: crash when throwing exception for NULL string. (Dhiraj
            Mishra)
Solution:   Handle NULL string like empty string. (closes #7139)

Problem:    Vim9: cannot use a {} block at script level.
Solution:   Recognize a {} block.

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

Problem:    Vim9: :let will soon be disallowed.
Solution:   Add v:disallow_let temporarily.  Fix tests.

Problem:    Vim9: using ":const!" is weird.
Solution:   Use "var" - "final" - "const" like Dart.  "let" still works for
            now.

Problem:    Vim9: no error if comma is missing in between arguments.
Solution:   Give an error message.

Problem:    Build failure due to missing error message.
Solution:   Add error message.