Allowing Developers to request specific codepoints to load

A comment came up on the CSS issue for adding incremental transfer to
supports asking if we had a mechanism to allow developers to request a
specific set of characters to load via incremental transfer:
https://github.com/w3c/csswg-drafts/issues/6063#issuecomment-797426308

This would likely be a useful use case to support. I think the easiest way
to add this would be through the javascript font loading api. Currently in
the load()
<https://developer.mozilla.org/en-US/docs/Web/API/FontFaceSet/load> call
you can specify text. Which is used to pick the relevant unicode ranges.
It's possible this could be extended to also pass the text down to the
actual font loading call where the incxfer client would ensure those
codepoints are in the set requested.

Thoughts?