Allow Language Server to be used as standalone process independent of VS Code Extension

[ferntheplant]

I was trying to setup the Markdoc Language Server for use with Helix (terminal based editor like nvim) but ran into issues. Installing the package with npm install -g @markdoc/language-server lead to no language server binary or script being accessible via the npm bin directory.

After investigating the source code I found it was because there was no build target for a standalone process running the language server. However, even after adding such a build target and installing the target to my npm global bin the server was not working. After digging around through the code I made 2 core discoveries:

1. The expected config the LSP client provides to the server requires redundant nesting (see README changes)
   • This is hidden from VS Code users as the extension generates the nesting from the VS Code config
2. The server code contained a bug while reading the config (see server/server.ts:32 changes)
   • I'm unsure how the VS Code extension is even functional as without my patch the server threw an error at line 48 while reading options.config as options was undefined due to a bug in the connect function

I don't think my PR follows best practices for building/bundling an npm package intended to be installed globally for it's binaries but I think it generally gets the idea across and can be used as a jumping off point.

[CLAassistant]

All committers have signed the CLA.

[rpaul-stripe]

This looks good to me. I think you accidentally left your package name in the package.json, but I'm happy to merge once that's resolved.

[rpaul-stripe]

Thank you!