Add option for labeling based on PR size

[jw-maynard]

Description: Adds a new action input that will enable labeling the PR based on the size. The size thresholds have default but are also configurable.

BREAKING CHANGE: The configuration format has been changed.

• Glob matching labels are now stored under the top level key label-config and remain otherwise unchanged.
• A new optional top level key named size-config has been added to hold a JSON object containing custom size labels.

Related issue: #486

Check list:

• [x] Mark if documentation changes are required.
• [x] Mark if tests were added or updated to cover the changes.

[jw-maynard]

I haven't added Doc updates yet, but if the code changes are acceptable LMK and I'll update the Docs and add it to this PR. Thanks!