Add accessor for runScript Build Phases on PBXTarget.

[michaelmcguire]

Resolves https://github.com/tuist/xcodeproj/issues/716

Short description 📝

PBXTarget has a set of accessors for the various types of build phases but is missing one for Run Script.

Solution 📦

This adds an access with tests.

Implementation 👩‍💻👨‍💻

• [x] Add accessor
• [x] Add tests

[kwridan]

@all-contributors add @michaelmcguire for code

[allcontributors[bot]]

@kwridan

I've put up a pull request to add @michaelmcguire! :tada: