margin-trim : link to Chromium/Firefox issues

[Ugoku]

This feature is implemented in Safari, and already listed on Caniuse.

• Chromium issue: https://bugs.chromium.org/p/chromium/issues/detail?id=1405835
• Firefox issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1506241

Please link these so people can express their interest more easily

[macx]

Missed margin-trim specially in the JSON files, used by Caniuse workflow for Alfred.

[Fyrd]

Currently margin-trim is only available via the MDN BCD data, so would need to either first add it as a caniuse feature or add support for more metadata on BCD data.