Add API for getting the source code of an entity

[amomchilov]

Given a declaration like a method, we should provide a #source_code method which returns a String? of its source code, trimmed to length.

[vinistock]

This would require remembering all sources in the graph or else in-memory sources that aren't committed to disk yet won't work, which could increase memory usage quite a bit.

[soutaro]

I'm wondering if something that associates Rubydex definitions and Prism node would make sense, not returning the source code directly.

• We want the method call node that corresponds to MethodReference, to implement something with its arguments
• We want the method definition node for MethodDefinition, to confirm it's method body has specific patterns
• We want the constant definition node that corresponds to ConstantDefinition, to get the value of the assignment (Expose the location range of a constant's assigned value #844)

With this API, we don't need to keep all of the source code (or add more locations to definitions). The library user keeps the text in the memory or takes care of the source code synchronization.

[vinistock]

Yeah, I think this warrants a deeper investigation into what fundamentals we need. Do we need to remember the exact original source code? Just extra locations? Or maybe do we need targeted information that some bitflags would suffice?