Introducing variable `@content-heading-font-family-generic`, that may be
used to specify an alternate font for `h1` and `h2` for languages which
render poorly using the standard heading font stack.
Set it for now on JA, HE and KO languages, based on reported issues.
Bug: T73240
Change-Id: I5217a7947427055f4cb61185d9d7d79848605dd3