formatter: render numeric character references like named ones

[gibson042]

Ref #488

[bakkot]

This will render some things it probably shouldn't - e.g.   (no-break space), which is visually indistinguishable from a regular space character in most editors.

I'd like to filter on the same regex as we use for regular entities: i.e. don't render entities which match /\p{White_Space}|\p{DI}|\p{gc=M}|\p{gc=C}/u. I can push a commit with that change sometime if you don't get to it before me.

(It might actually be nice to translate numeric forms of entities which we want to keep in entity form into their corresponding named entity, when they have a name - so &#A0; into   - but that doesn't need to happen in this PR.)