feat: change pp.natLit to be enabled by default

[eric-wieser]

This PR changes the default such that raw Nat literals always print with nat_lit.

This makes the experience of hovering over 37 : Nat in the infoview less confusing, as it no longer expands to a term that contains another 37 : Nat.

In practice, raw natural literals should never appear in user code, and if they do they are indicative of a buggy tactic or simp lemma that dropped the required OfNat.ofNat wrapper that literals are supposed to have. Enabling this pretty printer option by default increases the chance of these mistakes being reported by the users affected by such bugs, and will be invisible to users who are unaffected.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase b18f3a387772178544db849647684411886dbfd0 --onto 5240405cf4b481278db2bda3357bd2b8b8b466cd. (2024-12-24 00:58:08)

[kmill]

This makes the experience of hovering over 37 : Nat in the infoview less confusing, as it no longer expands to a term that reads 37 : Nat.

Did you consider having only the hover show nat_lit 37? We could do that by adjusting the options set in Lean.Widget.ppExprTagged or by perhaps making the natlit delaborator respect pp.explicit, if we expand its meaning a bit.

Changing the default value for pp.natLit does make sense to me, but given that its behavior was specified in RFC #3021, I think it would be good to have this change be backed by an RFC as well.