Add LSP implementation for Linefeed

Using the Language Server Protocol, syntax highlighting, autocomplete, error highlighting, and more would be supported immediately by multiple editors, including VS Code and Neovim.

Having the language server run in Rust would be the obvious choice, since you can just build on top of the existing parser and compiler. This way, updating the language server when the language changes also becomes extremely straight-forward.

First step is to get syntax highlighting working. Afterwards, autocomplete would be great, and error highlighting (e.g. for parser / compiler errors) would also make sense.

For highlighting:
- For VS Code, highlighting could be provided entirely via the LSP if we utilise semantic highlighting (see [the docs](https://code.visualstudio.com/api/language-extensions/semantic-highlight-guide)). In other words, no TextMate grammar would have to be specified - the existing parser and AST can just be utilised.
- For Neovim, highlighting can also be done via the LSP and semantic highlighting (see https://github.com/neovim/neovim/pull/21100).