PR Size Checker v3

feat!: Use main as the default target branch

This will be better for future compatibility as more repositories
migrate to `main` as the default branch.

BREAKING CHANGE: Repository's using `master` as the default branch will
have to explicitly set `default_branch: master` in their inputs.

PR Size Checker v2

feat(diff): use git diff so we can exclude *.lock files

Using `git diff` instead of the API response means that we can exclude particular file types - all *.lock files are now excluded.

PR Size Checker v1.1

Merge pull request #1 from S-Coyle/add_readme

Add readme and other documentation and base files