Introducing solargraph.

[steiley]

I am thinking that introducing solargraph is also good.
I tried implementing hover with solargraph in my repository.
The following is the change.

master...steiley:introduce_solargraph

I would be happy if you give me opinions.

[mtsmfm]

@steiley

Thank you for giving me your opinion!

I'm positive to introduce new tools to make the ruby language server better 👍
But I haven't used Solargraph so let me try it.

And sorry, currently I'm mainly focusing e2e test to make sure #44 is fixed.
It will take a while 💦