Problem:    The "num64" feature is available everywhere and building without
            it causes problems.
Solution:   Graduage the "num64" feature. (James McCoy, closes #5650)

Problem:    Vim9: optional function arguments don't work yet.
Solution:   Implement optional function arguments.

Problem:    Problems parsing :term arguments.
Solution:   Improve parsing, fix memory leak, add tests. (Ozaki Kiichi,
            closes #5536)

Problem:    Build fails with old compiler.
Solution:   Do not use anonymous unions. (John Marriott)

Problem:    Detecting a script was already sourced is unreliable.
Solution:   Do not use the inode number.

Problem:    Maintaining a Vim9 branch separately is more work.
Solution:   Merge the Vim9 script changes.

Problem:    Info about sourced scripts is scattered.
Solution:   Use scriptitem_T for info about a script, including s: variables.
            Drop ga_scripts.

Problem:    VAR_SPECIAL is also used for booleans.
Solution:   Add VAR_BOOL for better type checking.

Problem:    ETYPE_ is used for two different enums.
Solution:   Rename one to use EXPR_.

Problem:    Execution stack is incomplete and inefficient.
Solution:   Introduce a proper execution stack and use it instead of
            sourcing_name/sourcing_lnum.  Create a string only when used.