Further clarified config converters structure in code

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing issue: starts on Refactor common parts of convertConfig and convertEditorConfig into runFileConversion helper #274
• That issue was marked as status: accepting prs

Overview

Before we jump into the really meaty parts of the proposed refactor, there are a couple of small improvements that would help improve ease of code comprehension on their own:

• Standalone ConfigConverter type to use when referring to the array of functions passed to runCli that each do some kind of conversion
• Rename convertConfig to the more precise convertLintConfig, as it's used alongside convertEditorConfig

FYI @MrCube42 if you're interested 😊