Add ruby-lsp as an optional dependency

This provides an LSP interface to the codebase, allowing for easier
navigation, searching, etc. in editors that support LSP.

Author: duncan-bayne

Gemfile

@@ -14,6 +14,7 @@ gem "rspec", "~> 3.8"
 gem "rspec-wait"
 gem "rubocop", "~> 1.22"
 gem "rubocop-rake", require: false
+gem "ruby-lsp", require: false
 gem "sinatra", "~> 3.2"
 gem "yard", "~> 0.9", require: false