runtime: Remove fallback :CompilerSet definition from compiler plugins

The :CompilerSet command was added in version Vim 6.4 which was released
twenty years ago.  Other runtime files do not support versions of that
vintage so it is reasonable to remove this fallback command definition
now.

closes: vim/vim#14399

https://github.com/vim/vim/commit/408281e16a36c15eed10fbf0406fa8ab159fc4bf

Co-authored-by: Doug Kearns <dougkearns@gmail.com>

Problem:  Modula2 filetype support lacking
Solution: Improve the Modula-2 runtime support, add additional modula2
          dialects, add compiler plugin, update syntax highlighting,
          include syntax tests, update Makefiles (Doug Kearns)

closes: vim/vim#6796
closes: vim/vim#8115

https://github.com/vim/vim/commit/68a89470693c7687d4e736ca056c05de632e3ac7

- Luaify the detection script:

  - Split the `(*!m2foo*)` and `(*!m2foo+bar*)` detection into two Lua patterns,
    as Lua capture groups cannot be used with `?` and friends (as they only work
    on character classes).

  - Use `vim.api.nvim_buf_call()` (ew) to call `modula2#SetDialect()` to ensure
    `b:modula2` is set for the given bufnr.

- Skip the syntax screendump tests. (A shame as they test some of the detection
  from `(*!m2foo+bar*)` tags, but I tested this locally and it seems to work)

- Port the synmenu.vim changes from Vim9 script. (Also tested this locally)

- (And also add the missing comma for `b:browsefilter` from earlier.)

Co-authored-by: Doug Kearns <dougkearns@gmail.com>