Add solargraph to development dependencies

What does this MR do?

Add solargraph as project development dependency.

I believe this could improve usability for people using such IDE's like vscode or atom which rely on ruby language server which provides autocompletion and additional development experience improvements.

While this isn't strictly necessary and could be used with a standalone solargraph installation, having it defined in the project Gemfile allows starting the language server via bundler ensuring that docs are generated for the correct gems.

The gem itself is not required by the project thus it should not warrant any additional testing, though it does introduce some additional transitive dependencies.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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