Add support for folding defs

[RyanBrushett]

Add support for handling file contents and folding method definitions.

Implementation

• Add support for document opened, changed and closed events. We need those for every request. We basically keep a hash as storage where the key is the file URI and the value is the content of that file. Most other requests will only send us the URI and so we just access the hash to get the file contents
• Add basic support for folding method definitions

Tophat

1. Switch to this branch, bundle install
2. In vscode-ruby-lsp, click F5 to start running the extension in debug mode
3. On the extension debug window, open the ruby-lsp project itself
4. Verify that you can fold method definitions and collapse them into a single line

[RyanBrushett]

Not hella sure on the file structure. Could be a lot different of course.

[vinistock]

In the latest commit, I just refactored to use the language server gem's implementation of the folding ranges interface. We were re-defining it ourselves, which isn't necessary.