fix: A couple of bootstrapping hiccups related to Std.Do (#9030)

This PR fixes a couple of bootstrapping-related hiccups in the newly
added `Std.Do` module. More precisely,

* The `spec` attribute syntax was registered under the wrong name and
its implementation needed to use a different priority parser
* Elaborators and delaborators for `MGoal`, `Triple`, `PostCond` and
`PostCond.total` were broken and are now properly builtin
* `Std.Do` should not transitively import `Std.Tactic.Do.Syntax`

Co-authored-by: Sebastian Graf <sg@lean-fro.org>