* Setting `pp.mvars` to false causes metavariables to pretty print as
`?_`.
* Setting `pp.mvars.withType` to true causes metavariables to pretty
print with type ascriptions.
Motivation: when making tests, it is inconvenient using `#guard_msgs`
when there are metavariables, since the unique numbering is subject to
change.
This feature does not use `⋯` omissions since a metavariable is already
in a sense an omitted term. If repeated metavariables do not appear in
an expression, there is a chance that a term pretty printed with
`pp.mvars` set to false can still elaborate to the correct term, unlike
for other omissions.
(In the future we could consider an option that pretty prints uniquely
numbered metavariables as `?m✝`, `?m✝¹`, `?m✝²`, etc. to be able to tell
them apart, at least in the same pretty printed expression. It would
take care to make sure that these names are stable across different
hovers.)
Closes#3781
9cb114eb83