Manually set the support status of advanced attr()

[foolip]

There is no BCD entry for attr() working outside of the content property for pseudo-elements, so this is the only way right now.

[foolip]

I asked @tursunova about this and she found that I'm just wrong here. https://bugs.webkit.org/show_bug.cgi?id=26609 and https://bugzilla.mozilla.org/show_bug.cgi?id=435426 remain open, so only Chromium supports attr() as defined by attr.yml here in web-features:

The attr() CSS function, in the context of any property, returns the value of an attribute of an HTML element, with the option to return it as a specific type or unit. You can set a default value for missing or invalid attributes.

Since there doesn't seem to be anything in BCD matching what we need, I guess we'll need to write a custom support statement.

[foolip]

I've updated this PR to manually set the status. But @tursunova also asks if we should drop <fallback> here? None of the BCD entries here correspond to the important behavior that developers have been asking for, so I could see the argument for making this a BCD-free entry. @ddbeck WDYT?