Add Helix installation instructions

[philipgiuliani]

Adds the helix installation instructions. I am using it myself in Helix and it works like a charm.

Closes #228

[scottming]

Hi, @philipgiuliani I tried this on my mac, it won't works.

Bad language config: unknown field `language-servers`, expected one of `name`, `scope`, `file-types`, `shebangs`, `roots`, `comment-token`, `text-width`, `soft-wrap`, `config`, `auto-format`, `formatter`, `diagnostic-severity`, `grammar`, `injection-regex`, `language-server`, `indent`, `debugger`, `auto-pairs`, `rulers`, `workspace-lsp-roots`
in `language`

Press <ENTER> to continue with default language config

and tried

[language-server.lexical]
command = "/Users/scottming/Code/lexical/_build/dev/package/lexical"

[[language]]
name = "elixir"
language-server = ["lexical"]

[[language]]
name = "heex"
language-server = ["lexical"]
``` too

[philipgiuliani]

Oh sorry, the last Version of Helix has been released on May. I am using the next version, which will be released within September: helix-editor/helix#8086

Unfortunately, there was a breaking change with the last Helix release. Should I document both configurations here, or maybe we should just extend the information that it only works above Helix 23.09?

From the Changelog:

Breaking changes:

• Support multiple language servers per language (#2507)
  • This is a breaking change to language configuration

[scottming]

extend the information that it only works above Helix 23.09?

I think adding this is enough.

[scottming]

Ok, I tried the main branch, and this config works, thanks.