@next/font/google: scope font names (#3049)
Fixes WEB-341
This incorporates a digest of the font request into the name of the
font-family defined in the stylesheet, used in the css class, and
returned to the JavaScript importer. This makes font-family names
returned from `@next/font/google` in stylesheets unguessable.
Note that this implementation uses 8-character long hex digests compared
to the 6-character version in Next.js+webpack.
Test Plan: imported Open Sans as a symbol, verified that stylesheet is
updated to reference both `@font-face` definitions and the css class
rules with `font-family: '__Open_Sans_3b020fa1';`. Verified that
exported js object `style.fontFamily` references the same.
