test parser support for underscore curry

[simeonschaub]

This is an initial implementation of the solution proposed in
#24990 (comment).
This is only the parser part to that, so it doesn't currently do
anything useful, but I am mainly putting this up so we can run PkgEval
on it to see how breaking this solution would be.

[simeonschaub]

Looks like Documenter doesn't like this already

[simeonschaub]

@nanosoldier runtests(ALL, vs = ":master")

[nanosoldier]

Your package evaluation job has completed - possible new issues were detected. A full report can be found here. cc @maleadt

[simeonschaub]

991 packages failed tests only on the current version.

GitHub won't even display the file... 😢

[simeonschaub]

It does seem like most failures are coming from transitive dependencies on MacroTools, but this might still be a little bit too breaking. Some initial analysis:

 (945, MacroTools)
 (38, MatchImpl)
 (30, Unknown)
 (28, SyntaxError)
 (6, Match)
 (3, MatchCore)
 (3, PrintingChange)
 (1, CBinding)
 (1, NewExceptionType)
 (1, DocTest)

[simeonschaub]

Marking this for triage to decide whether this is the right approach regarding #24990 vs. only implementing that in lowering and if so, whether this has to be considered a breaking change, meaning it will have to wait for 2.0

[vtjnash]

Triage said this doesn't appear to really work. We also discussed that there isn't agreement about what rule to use for #24990