Problem:    Vim9: can create a script variable from a legacy function.
Solution:   Disallow creating a script variable from a function.

Problem:    Vim9: allowing use of "s:" leads to inconsistencies.
Solution:   Disallow using "s:" in Vim9 script at the script level.

Problem:    Vim9: line number of exception is not set.
Solution:   Set the line number before throwing an exception. (closes #9755)

Problem:    Test fails where lines are skipped.
Solution:   Only give an error when not skipping commands.

Problem:    No error for using :vim9script in a :def function.
Solution:   Give an error when compiling.

Problem:    Vim9: not all code covered by tests.
Solution:   Add a couple more tests.

Problem:    Vim9: type of item in for loop not checked properly.
Solution:   Adjust the type checking. (closes #9683)

Problem:    Vim9: can use old style autoload function name.
Solution:   Give an error for old style autoload function name.

Problem:    Vim9: finding global function without g: prefix but not finding
            global variable is inconsistent.
Solution:   Require using g: for a global function.  Change the vim9.vim
            script into a Vim9 script with exports.  Fix that import in legacy
            script does not work.

Problem:    E464 does not always include the offending command.
Solution:   Add another error message with "%s". (closes #9564)