Add `letter-spacing` support for percentage values

[benface]

Safari already supports it: https://bugs.webkit.org/show_bug.cgi?id=262762
Chromium bug: https://issues.chromium.org/u/1/issues/327740939
Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1884118