Require bug link for partial implementation

[caugner]

What type of issue is this?

Linter issue

What is the issue?

When a browser is marked as partially supporting a feature, we don't currently require a bug link, making it hard to check if the limitation was meanwhile resolved.

What behavior were you expecting?

For every support statement with partial_implementation: true, we should require a bug link, either in the notes (preferred) or in the impl_url.

What version(s) of BCD is the issue present in?

• [ ] The current BCD release
• [ ] The current version of the main branch

Do you have anything more you want to share?

The following Firefox support statement for css.selectors.highlight prompted this:

https://github.com/mdn/browser-compat-data/blob/9ec9ff9b03e756da2b1b906c9ef00334775453aa/css/selectors/highlight.json#L21-L25

Requiring a bug link would provide context, make it easier to check if the limitation is still valid, and identify the browser version in which it was fixed.