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>