Use activated gem paths to register document selectors

[vinistock]

Motivation

Closes #2709

We were using the environment variable GEM_PATH to register the LSP client's document selector, so that people can use the Ruby LSP's features inside gem source code. Unfortunately, that won't work for shim based version managers because they don't actually set that environment variable, so we never register the document selector and navigation is unavailable.

Implementation

I recommend reviewing per commit:

1. The first one starts returning the Gem.path from our activation scripts. Even for shim based version managers, that method is still going to return the array of directories where gems should be installed
2. Then we start using the new gemPath field to register the document selector instead of relying on the environment variable

Automated Tests

Added a new test.