Add find references support for constants

[vinistock]

Motivation

Closes #202

With the same infrastructure added in #2626, we can easily support finding constant references. Instead of building an array of text edits, we just return an array of locations.

Implementation

The idea is to use the same reference finder implementation and then return all of the hits back as locations.

I also had to fix a small duplication issue to be able to respect the LSP spec's includeDeclaration parameter.

Automated Tests

Added tests.

[andyw8]

Do we need any safeguards for someone running Find References on a large codebase for some constant with a high number of occurrences?

[andyw8]

Very exciting to have this.

[andyw8]

I found one issue:

Create a new file, but don't save it:

class Widget2
  def bar
  end
end

Widget2

Right-click on Widget2 in the first line then Find All References

The LSP will error with:

Ruby LSP error NoMethodError: undefined method `full_name' for an instance of Prism::ProgramNode /Users/andyw8/src/github.com/Shopify/ruby-lsp/lib/ruby_lsp/requests/support/common.rb:152:in `constant_name'

[vinistock]

The issue was related to not finding a target. I fixed it in the latest push.