Add documentation for Helix editor

[EskelinenAntti]

Add documentation on how to setup the LSP with Helix editor.

[Copilot]

Pull Request Overview

This PR adds documentation for setting up the Kotlin Language Server Protocol (LSP) with the Helix text editor. The documentation provides step-by-step instructions for configuring Helix to work with the Kotlin LSP using the stdio communication method.

Key changes:

• Creates a new documentation file specifically for Helix editor setup
• Provides detailed instructions for downloading, installing, and configuring the Kotlin LSP
• Includes TOML configuration examples for Helix's language server setup

[bartekpacia]

Hey @EskelinenAntti, thank you for your contribution.

I downloaded Helix and followed this guide, but unfortunately, Helix doesn't connect to the Kotlin LSP.

Maybe I'm doing something wrong (never used Helix before), but I'd like to make sure the instructions are correct before we merge them.

[EskelinenAntti]

Hi @bartekpacia! What do you get if you run

hx --health kotlin                                                                                                                                                       main

For me (latest versions from homebrew Helix 25.07.1 + Kotlin LSP 0.253.10629) and fresh config with nothing but copied ~/.config/helix/languages.toml from the PR documentation it outputs:

Configured language servers:
  ✓ kotlin: /opt/homebrew/bin/kotlin-lsp

I also need to wait couple of seconds before the LSP can be used, but after the initial delay go-to-definition gd and go-to-references gr key mappings start to work.