Add `invalid` to LABELS.md

[keithamus]

This adds the invalid label, to the LABELS.md.

The motivation for such a label is to properly mark these issues as spam. The specific invalid label is one of the signals used by spam detection algorithms at GitHub which allows us to more easily surface spam issues to the spam team.

[annevk]

You need to add it to labels.json. You can then generate the .md file by running the corresponding Python script. And I can then run it after landing this to update the labels everywhere.

[keithamus]

Okay I've done just that, thanks!

[domenic]

Is "spam" not a good enough label? https://github.com/whatwg/html/pulls?q=label%3Aspam

[keithamus]

It's not one of the signals because it's a custom label. invalid is a default label that comes with all repositories and is used as a signal. We'll look at adding spam to the list of signal labels, but for now it would be great if we could use invalid.