This PR is a bit of a last minute change, but I realized there was
actually a third player in the Elixir LSP space who wants support as
well,
[lexical](https://github.com/zed-industries/community/issues/1567). I
realized that the settings arrangement I shipped in this preview
precludes adding a third kind of LSP. I don't have the time to learn how
to fully support this LSP, but I thought I'd at least refactor how the
settings are represented before this hits stable.

Release Notes:

- Changed the new `"elixir": { "next": "on" }` setting to `"elixir": {
"lsp": "next_ls" }`. The `elixir.lsp` setting requires a full restart to
take effect. (Preview only)