Add EmmyLua_LS support

[notpeter]

Add support for emmylua_ls using the same pattern as the Zed Ruby extension to support multiple LSPs (distinct IDs, settings, paths, etc). Only LuaLS will be enabled by default. While making these changes I also improved LSP Symbol and Completion handling for both the existing and new language servers.

• Add support for emmylua_ls in addition to lua-language-server
• Support config specified binary/args/env; PATH found binary with worktree.which; Zed auto-download from GitHub releases (both LSPs)
• Keep managed downloads isolated by language server
• Preserve server-provided metadata in custom completion labels. Previously, custom labels omitted signatures, inferred types, and source descriptions.
• Handle PROPERTY completions same as existing FIELD completions (both LSPs)
• Highlight symbols by LSP kind without parsing their labels
• Improve README.md with advanced configuration examples and links

Prior to shipping an extension in the Zed extension store with this code, a companion PR to the Zed repo with new default settings is needed so Zed will automatically prefer LuaLS and disable EmmyLuaLS by default (merge, cherry-pick to preview; wait a week to minimize impact):

    "Lua": {
     "language_servers": ["lua-language-server", "!emmylua_ls", "..."],
    },