chore: remove @ from rw? suggestions, and enable hover on constants in #check (#3911)

* Replaces the unused `Lean.PrettyPrinter.ppConst` with
`MessageData.ofConst` (which similarly avoids an unnecessary `@`) and
that further generates a hover for the constant

* Uses this in `TryThis.addRewriteSuggestion`, so that `rw?` suggestions
don't have unnecessary `@`s.

* Add `MessageData.signature`, as a wrapper around
`PrettyPrinter.signature`, using the same machinery to generate hovers
for constants, improving the hover behaviour in #check so that we get
second order pop-up for constants in the signature. (Not sure how to
write tests for second order hovers, so there is no test for this.)