Add CSS filter() function

[nt1m]

Fixes #3378

This has been supported since Safari 9, see https://caniuse.com/css-filter-function.

[nt1m]

@captainbrosset @foolip Could you please take a look?

[captainbrosset]

compat data are not currently available, but will be linked once they are.

@nt1m do you know if there's already a PR opened on BCD to add the required keys for this feature? If there isn't one, would you or someone on your end be able to create one?
We can of course merge the new feature without BCD keys in the meantime, but if a BCD PR already exists (or is quick to create), it would be better to wait for it. This way, less risks of forgetting to update the feature once the keys are available.

[nt1m]

I created a BCD PR here: mdn/browser-compat-data#28916 . I was reluctant to create one until some MDN docs were written, but it looks like you can just create one independently!

[captainbrosset]

Ah, you'll need double quotes around the descriptions. Otherwise the : characters break the yaml syntax.

[captainbrosset]

We'll have to wait for the BCD key to be released and then bump our dependency to BCD on the repo for this to build.