Make LSP configurable using initialization options

[bioball]

This defines initialization options that clients can send during the initialize request, to configure the behavior of the server.

This also introduces extended capabilities that clients can notify the server about.

Also:

• Switch some messages from sendActionableNotification to showMessageRequest, which is a built-in message.
• Look for Pkl CLI on $PATH if not configured by client.
• Render doc comments as normal URIs instead of command:pkl.open.file, unless configured to do so.