Consider adding sizing labels for pull requests

[simonwuelker]

https://github.com/mdn/browser-compat-data has different labels that are automatically applied to pull requests based on their size.
For example, a PR that updates 0-6 LoC will get the size:xs label, whereas one that updates 25-100 LoC will get size:m.

These labels serve two purposes:

• They can hint to the author that the PR should be split up into smaller changes (size:xl)
• They (roughly) indicate to reviewers how much effort it is to review the PR, without first having to click on it

I think these labels are useful and it would be nice to have the highfive bot apply them to servo PRs. Might be a bit noisy though, we would have to see.

[jdm]

Definitely a neat idea!

[jdm]

The pull_request object includes these fields:

    "commits": 2,
    "additions": 72,
    "deletions": 10,
    "changed_files": 2

[TimvdLippe]

Can we ignore WPT changes for this? E.g. would be a shame if a tiny fix with loads of WPT tests fixed would be marked as large, which discourages reviews.

[jdm]

Good point!