Vim9: :def in legacy script can create a script variable - #21379

Problem:  A :def function in a legacy script can create a script-local
          variable by assigning to it, while declaring one with "var"
          gives E1101.
Solution: Give E1089 for assigning to an unknown script-local variable
          in any :def function, not only in one defined in a Vim9
          script.  Remove uf_script_ctx_version, which was only used
          for this check.  Do not give it with ":source ++dryrun".

runtime/menu.vim created s:bmenu_short, s:bmenu_count, s:suglist,
s:fromword, s:changeitem, s:additem and s:ignoreitem this way, define
them at the script level.

":source ++dryrun" does not execute ":let" in a legacy script, thus
every script variable would be unknown when compiling.  Document that
reading or calling such a variable without "s:" still gives an error,
and separate the paragraphs of ":source-dryrun" with blank lines.

Test_unlet() created s:somevar this way, define it at the script level
instead.  Test_using_s_var_in_function() checks that assigning to
s:scriptlevel in a :def function of a legacy script gives E1089 when
the variable does not exist.  Test_source_dryrun() checks that
assigning to a script variable with "s:" in a :def function of a
legacy script gives no error with ":source ++dryrun".

Assisted-by: Claude
Signed-off-by: Hirohito Higashi <[email protected]>