[NFC] Tighten checking for PBD introducers

Treating introducers as keywords is now always conditional on whether `shouldParsePatternBinding(introducer:)` returns `true`. That method has also been modified to correctly handle an edge case with wildcard patterns.

Author: beccadax

Sources/SwiftParser/Declarations.swift

@@ -140,6 +140,8 @@ extension TokenConsumer {
 
       // Otherwise, parse it as an expression.
       return false
+    case .rhs(let introducer):
+      return subparser.shouldParsePatternBinding(introducer: introducer)
     case .some(_):
       // All other decl start keywords unconditionally start a decl.
       return true

Sources/SwiftParser/Patterns.swift

@@ -272,7 +272,10 @@ extension Parser.Lookahead {
     // than likely need to be made contextual as well before finalizing their
     // grammar.
     case ._borrowing, .borrowing:
-      return peek(isAt: TokenSpec(.identifier, allowAtStartOfLine: false))
+      return peek(
+        isAt: TokenSpec(.identifier, allowAtStartOfLine: false),
+        TokenSpec(.wildcard, allowAtStartOfLine: false)
+      )
     default:
       // Other keywords can be parsed unconditionally.
       return true