rhombus support

[jyn514]

(rhombus is https://github.com/racket/rhombus/; lang docs at https://docs.racket-lang.org/rhombus/index.html)

@benknoble had the following suggestions on discord:

• create a new ftplugin/rhombus.vim that tweaks :setlocal commentstring and b:undo_ftplugin. vim also suggests setting b:undo_indent; see :h ftplugin.
• add .rhm to

  vim-racket/ftdetect/racket.vim

  Line 66 in 0ca759e

  au BufRead,BufNewFile *.rkt,*.rktl,*.zuo call RacketDetectHashLang()

• document how to set up an LSP for Rhombus. i see ALE support for racket in

  vim-racket/ftplugin/jsond.vim

  Line 21 in 0ca759e

  let b:ale_linter_aliases = ['racket']

  and

  vim-racket/ftplugin/racket-info.vim

  Line 73 in 0ca759e

  let b:ale_linter_aliases = ['racket']

  for neovim + lspconfig i have this configuration for myself; maybe it will help someone:

local lsplang = require('lspconfig.configs')
if not lsplang.rhombus then
	lsplang.rhombus = {
		default_config = {
			cmd = {"racket",  "-l", "racket-langserver"},
			filetypes = { "rhombus" },
			root_dir = vim.fs.dirname,
			settings = {},
		},
	}
end
lspconfig.rhombus.setup {}

Note that there are some limitations to this setup.

• syntax highlighting is only enabled through the LSP, and the LSP has some bugs (in particular it doesn't do SematicTokens for keywords). that should probably be an upstream bug.
• autoindentation isn't automatic. we could fix that with some manual work in rhombus.vim. i think the recommended way is indent-expression?

[benknoble]

Now that Rhombus is officially supported by the LSP, that should make configuring the LSP to start in Rhombus files even better (and more urgent for this plugin).