Find files using a specific method on a given class

[12joan]

This PR extends the used_by.rb script with the ability to find all files referencing a specific method on the given script. This includes other C# scripts calling this method, in addition to any Unity scenes or prefabs containing UnityEvents referencing this method.

It may return some false positives, but it should return few or no false negatives.

Example

The basic usage of used_by.rb reports four files that reference the FireBullets script.

$ ruby used_by.rb Assets/src/FireBullets.cs
Found 4 file(s) using FireBullets.cs:
  Fire Bullets.prefab
  Turret.prefab
  MentoeHologramBulletsAttackAI.cs
  Turret.cs

If we want to filter these to return only methods that call the BeginFiring method on FireBullets, we can add this as an additional command-line argument.

$ ruby used_by.rb Assets/src/FireBullets.cs BeginFiring
Found 1 file(s) using the BeginFiring method of FireBullets.cs:
  Turret.cs

#no-test

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