[CSS] Add support for layer() option to @import

[Rumyra]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• mdn/data updated
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
  or
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Features to document

More implemented so it looks like we're good to go https://bugzilla.mozilla.org/show_bug.cgi?id=1699215 and https://bugzilla.mozilla.org/show_bug.cgi?id=1699220

Add support for layer() option to @import - this page https://developer.mozilla.org/en-US/docs/Web/CSS/@import but might want to check learn etc...

NB currently in experimental features

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1699217

[rachelandrew]

This relates to https://developer.mozilla.org/en-US/docs/Web/CSS/@layer which mentions the @import method.

[Rumyra]

Needs

• data
• bcd
• bcd @import - add layer()
• @import update
• @layer checked (but looks good)
• @layer needs modification (see related issue)
• raise bug that FF does not honour correct cascade when using @layer
• I think data needs a type, as well as the at-rule
• rel notes
• needs layer() page
• bcd
• revert-layer keyword (Assuming this actually changes a bunch of pages) -> this is going to change a bunch of pages, so I'm pulling it out into a separate issue.

[Rumyra]

Also related #9656

[Rumyra]

@dipikabh - after you've done the revert layer stuff do you want to take the last task on this issue - the @import page needs updating to include the layer function. To be honest the whole @import page could do with a review

[dipikabh]

@Rumyra, yes, I'll get started on this.

[dipikabh]

1. Documentation update:

• Add @layer syntax and examples to @import #15205

2. Other related updates:

• Remove experimental banner from @layer #14967
• Add syntax and description to @layer #15215

3. Linked bugs are already set to to dev-doc-complete.
4. Release note looks updated for layer() support in @import.