gems: add solargraph gem

Add the Solargraph gem to enable Ruby language integration for
supporting text editors.

This enables various features, including code completion, inline
documentation, type checking, syntax checking, linting, and jump to
definition/references.

Usage:

  Install Solargraph extension for your editor (see https://solargraph.org/guides)
  Run `bin/solargraph download-core` to install core Ruby language docs.
  Run `bin/solargraph bundle` to install Ruby gem docs.

See also:

* https://github.com/castwide/solargraph
* https://solargraph.org/guides/getting-started
* https://marketplace.visualstudio.com/items?itemName=castwide.solargraph

Gemfile

@@ -82,6 +82,7 @@ group :development, :test do
   gem 'pry-byebug'
   gem 'pry-rails'
   gem 'listen'
+  gem 'solargraph'
 end
 
 group :test do