[css.types.url] Add src() function

[ExE-Boss]

Summary

This adds initial compatibility data for the new CSS Values 4 src(…) function, which is like the url(…) function, except it doesn’t support the unquoted legacy syntax, so that src(var(‑‑foo)) is valid.

Test results and supporting details

• Spec URL: https://drafts.csswg.org/css-values/#urls
• Firefox bug: Bug 1707923 - [css‑values] Support the src() function in the CSS <url> type

[caugner]

Thank you for your PR.

However, BCD's data guidelines state:

Browser features that have not been implemented (or planned to be implemented) in any browser, should not be added to BCD.

We cannot consider the Firefox bug you referenced (and created yourself in 2021) as evidence for intent to implement, as the bug is unassigned and only has severity S3.

So I'm going ahead and close this PR.

Please feel free to reopen this when any browser shows intent to implement.

[ExE-Boss]

The fact that this isn’t listed anywhere else as unimplemented tripped me up when I tried to use src(…) in my newer stylesheets. (took a while to debug as @import fails aren’t logged anywhere in the dev tools)

It should probably be marked as such in bcd‑content for Web/CSS/<url> § src().