Feat/fst order pred part3 & part4 (#457)

* support wildcard predicates in frontend

* suport wildcard predicate in podlang

* add validation test

* test full flow and apply some fixes

* fix clippy

* fix merge issues

* use desugared predicate

* Fix parsing of intro statement templates inside custom predicates

* Tidy up comments

* lang: handle wildcard predicate

* add unreachable message

---------

Co-authored-by: Rob Knight <mail@robknight.org.uk>

src/lang/error.rs

@@ -88,6 +88,9 @@ pub enum ValidationError {
         first_span: Option<Span>,
         second_span: Option<Span>,
     },
+
+    #[error("Wildcard '{name}' collides with a predicate name")]
+    WildcardPredicateNameCollision { name: String },
 }
 
 /// Lowering errors from frontend AST lowering to middleware