Consider removing Nokogiri dependency from the project

[jvcarli]

Hi, thank you for the project!

I am restricted to use a Ruby version built with MSVC compiler (x64-mswin64_140) and unfortunatelly compiling Ruby gems that have native extensions is really problematic when working with mswin builds.

I see a mention of removing Nokogiri dependency in commit a54b3a2, and see that currently it is only being requierd directly by one file: https://github.com/castwide/solargraph/blob/master/lib/solargraph/converters/dd.rb . Would you consider removing it?

I don't have much Ruby experience and don't know about Solargraph internals but I'd be willing to help because I already had to patch a transitive dependency of my project (tonytonyjan/jaro_winkler@1c80dbf), that itself is a dependency of Nokogiri and I am in the process of patching Nokogiri itself just to install Solargraph (Currently I do some ugly hacks to install Nokogiri).

I still don't think patching Nokogiri, even if changes are merged upstream, is sustainable because Nokogiri has a history of being broken on mswin builds. You can check it on: https://github.com/sparklemotion/nokogiri/issues?q=msvc

Removing Nokogiri dependency would make things really smooth for people who wants to use Solargraph and needs to use Ruby mswin builds.

[castwide]

Nokogiri is a transitive dependency of reverse_markdown, which is responsible for converting HTML in documentation to markdown for display purposes. It's necessary because the language server protocol only supports plain text and markdown in documentation properties. A lot of extant documentation uses inline HTML, including the core and stdlib documentation in RBS.

I'm open to a pure Ruby solution if we can find a way to retain reasonable documentation formatting.