This PR modifies the pretty printing of raw natural number literals; now
both `pp.explicit` and `pp.natLit` enable the `nat_lit` prefix. An
effect of this is that the hover on such a literal in the Infoview has
the `nat_lit` prefix.
Amendment to RFC #3021: In the reference-level explanation, now it
should read
> When `pp.natLit` and `pp.explicit` are false, then the `nat_lit n`
expression delaborates as `n`, and otherwise it delaborates as `nat_lit
n`.
