New config structure for labeler

[joshdales]

Description:
Start of a new config structure for the labeler action - it no longer defaults to checking paths, you need to specify changed-files in your labeler config file. Also adds head-branch and base-branch options for adding labels based on those branch names.

Also moves all the branch checking related logic into it's own file and adds unit tests for that.

This is what the labeler.yml will end up looking like after these changes:

label:
  - changed-files:
    - any: ['list', 'of', 'globs']
    - all: ['list', 'of', 'globs']
  - base-branch: ['list', 'of', 'regexps']
  - head-branch: ['list', 'of', 'regexps']

---

🤔 I'm trying not to refactor more of this than I need to - but I wonder if this as it is is potentially going to make things a little bit more confusing for a while, partly because it was all written around the sole option of file changes and now that is not the only way to use this action. So there are things like globs being referenced in a lot of function names which is kinda weird now.

And because we have nested options now and not each matchconfig individually in it's own item as a part of the map I'm reversing the conditions of checkMatch so that if any condition is successful then we apply the label rather than checking for all the options and if any fails then return.