feat: add pretty printer flag to instantiate non-ground delayed assignments

[sgraf812]

This PR adds an opt-in pretty printing flag pp.mvars.delayedNonGround that takes effect when pp.mvars.delayed is false.
Recall that instantiateMVars does not instantiate a delayed assigned MVar if the assignment is non-ground (i.e., contains metavariables itself). This flag opts into doing the instantiation while pretty printing. Example:

set_option pp.mvars.delayedNonGround false in -- the default
/--
error: Invalid match expression: The type of pattern variable 'a' contains metavariables:
  ?m.12
---
info: fun x => ?m.3 : ?m.12 × ?m.13 → ?m.12
-/
#guard_msgs in
#check fun (a, b) => a

set_option pp.mvars.delayedNonGround true in -- opt in
/--
error: Invalid match expression: The type of pattern variable 'a' contains metavariables:
  ?m.12
---
info: fun x => sorry : ?m.12 × ?m.13 → ?m.12
-/
#guard_msgs in
#check fun (a, b) => a

This flag is most useful for debugging metaprograms that create synthetic opaque metavariables and assign those to non-ground solutions. It happens regularly that incomplete instantiation leads debugging sessions down the wrong path.