Add support for a configuration file

[axelson]

This is in addition to the workspace/didChangeConfiguration option.

Could look at https://github.com/erlang-ls/erlang_ls/pull/61 for inspiration

[znorris]

I've run into a case where I believe adding support for a configuration file would prove helpful.

I recently resolved an issue where :yamerl was incorrectly throwing an error while I was debugging my application. The fix was adding an excludedModules key to the Elixir LS configuration. (Thanks for having the note about NIFs in the README)

"excludeModules": [
    ":yamerl"
]

Being able to exclude this any time the project is running Elixir LS would be very helpful as it will give a deliberate place to share this knowledge among other developers.

I'm using the VS Code extension which allows me to save the exclusion in our repo via the .vscode/launch.json file. However, the exclusion has to be duplicated among VS Code launch configurations, and obviously it won't be available to non VS Code devs.

Thanks for what you do! 🥳