Add support for Emacs configuration to `lune setup`

[kennethloeffler]

Right now, lune setup can only configure VS Code. There are a couple TODO comments mentioning a desire to support other editors:

https://github.com/lune-org/lune/blob/94ba331ed0025b45b687a401e06521afbc4ff109/src/cli/setup.rs#L27-L39

https://github.com/lune-org/lune/blob/94ba331ed0025b45b687a401e06521afbc4ff109/src/cli/setup.rs#L84-L99

I've recently created an integration for the Luau Language Server for Emacs at https://github.com/kennethloeffler/eglot-luau, and using this package to support the editing of Lune scripts is an important use case. I'd like to add support for Emacs configuration to lune setup so that users can simply run this command in their projects, and don't have to know all the idiosyncrasies of LSP workspace configuration for Eglot (Emacs' native implementation of an LSP client) to get it working for their Lune scripts.

While it's possible for users to do this setup themselves, it's yet more to learn before the package can be applied to this use case, and it's somewhat fiddly. The eglot documentation even mentions it as a caveat:

For some users, setting eglot-workspace-configuration is a somewhat daunting task. One of the reasons is having to manage the general Elisp syntax of per-mode directory-local variables, which uses alists (see Association Lists in GNU Emacs Lisp Reference Manual), and the specific syntax of Eglot’s variable, which uses plists.

Is Eglot-specific configuration something that fits into lune setup? If so, I'd love some pointers on how to best fit it into the existing code!