Support symbol procs in definition

[vinistock]

Motivation

Allow jumping to the definition of method calls passed as symbol proc arguments (e.g.: map(&:to_i)).

Implementation

1. Started accepting Prism::BlockArgumentNode as a possible target to locate
2. Started handling that node type in the definition listener
3. Made a small refactor to make the method definition helper more reusable

Automated Tests

Changed the example from #1981 to assert the right positions.

[andyw8]

I verified it also works when the symbol-to-proc isn't the only argument, e.g. values.inject(0, &:foo).