Add LSP For RIDDL

[reid-spencer]

We need development tools to be able to work with RIDDL and provide advanced editing features. That’s what LSP, Language Server Protocol is for. LSP has a JVM implementation in lsp4j with these coordinates:

• https://github.com/eclipse-lsp4j/lsp4j
• val lsp4j = "org.eclipse.lsp4j" % "org.eclipse.lsp4j" % "0.20.1"

Support in IntelliJ IDEA is documented here: https://plugins.jetbrains.com/docs/intellij/language-server-protocol.html

This was used to implement the Scala LSP. Should be good enough for our purposes.