Problem:    Vim9: no need to keep all script variables.
Solution:   Only keep script variables when a function was defined that could
            use them.  Fix freeing static string on exit.

Problem:    Vim9: variables declared in a local block are not found in
            when a function is compiled.
Solution:   Look for script variables in sn_all_vars.

Problem:    Vim9: function defined in a block can't use variables defined in
            that block.
Solution:   First step: Make a second hashtab that holds all script variables,
            also block-local ones, with more information.

Problem:    Vim9: can assign wrong type to script dict. (Christian J.  Robinson)
Solution:   Check the type if known.

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

Problem:    Vim9: internal error when using variable that was not set.
Solution:   Give a meaningful error. (closes #6937)

Problem:    Vim9: result of && and || expression cannot be assigned to a bool
            at the script level.
Solution:   Add the VAR_BOOL_OK flag.  Convert to bool when needed.

Problem:    Vim9: error for argument type does not mention the number.
Solution:   Pass the argument number to where the error is given.

Problem:    Vim9: white space checks are only done for a :def function.
Solution:   Also do checks at the script level.  Adjust the name of a few
            error messages.

Problem:    Vim9: error for an autocmd defined in a :def function in legacy
            Vim script.
Solution:   Don't check the variable type. (closes #6758)