Possibility of moving syntax definitions into a separate repository

There are some other language support extensions for different editors. Some of those editors directly ([Visual Studio Code][1]) or indirectly ([Visual Studio Code][2] or [Atom][3]) can use `.tmLanguage` syntax files.

Currently there are multiple Elixir extensions for those editors:
- Visual Studio Code: [vscode-elixir](https://github.com/fr1zle/vscode-elixir), [vscode-elixir-ls](https://github.com/JakeBecker/vscode-elixir-ls), [elixir-language-support](https://github.com/maxmellen/elixir-language-support)
- Atom: [ide-elixir](https://github.com/JakeBecker/ide-elixir), [language-elixir](https://github.com/elixir-editors/language-elixir) (maintained by this same organization)

Each of those packages currently maintains its own, incompatible and different syntax definition.

It could be possible to split the current `.tmLanguage` grammars into their own repository that could then be reused by different editor support packages (including this one).

The good part is that there would be a single source of truth for the Elixir grammar definition across editors, unifying the maintenance efforts.

I could make the changes myself and submit a PR, but I prefer to discuss it first to see if it would make sense.

[1]: https://code.visualstudio.com/api/language-extensions/syntax-highlight-guide#converting-an-existing-textmate-grammar
[2]: https://code.visualstudio.com/api/language-extensions/syntax-highlight-guide#using-yaml-to-write-a-grammar
[3]: https://flight-manual.atom.io/hacking-atom/sections/converting-from-textmate/#converting-from-textmate