Add rename support for constants

[vinistock]

Motivation

Closes #57

This PR adds rename support for constants, modules and classes.

Implementation

1. Created a ReferenceFinder. This is essentially a simplified copy of our declaration listener. I believe that either the declaration listener can be modified to support collecting references or we can extract some abstraction for both to use. However, I would like to start with a copy, so that we can see the patterns emerge as we add support for more renames
2. Then I added the rename request

Automated Tests

Added tests.

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[andyw8]

Investigating a hang with previews:

[andyw8]

Should we allow people to change namespaces with this? e.g. renaming Foo to Bar::Foo

It 'works' but does complicate some things.

[andyw8]

🤔 If a constant is already defined, should we prevent someone to renaming to that constant?

[vinistock]

🤔 If a constant is already defined, should we prevent someone to renaming to that constant?

Added a check for this. Great catch.

[st0012]

I still prefer assigning attributes to ivars. But it's a small detail that can be discussed later.