This PR adds the pretty printer option `pp.mdata`, which causes the
pretty printer to annotate terms with any metadata that is present. For
example,
```lean
set_option pp.mdata true
/-- info: [mdata noindex:true] 2 : Nat -/
#guard_msgs in #check no_index 2
```
The `[mdata ...] e` syntax is only for pretty printing.
Thanks to @Rob23oba for an initial version.
Closes#10929
