Add error case for unknown '(*...'

Missed this when implementing the rest of the
group kinds. Because we don't backtrack, we should
throw an error here after consuming a `*` in a
group.

Author: hamishknight

Sources/_MatchingEngine/Regex/Parse/LexicalAnalysis.swift

@@ -516,6 +516,8 @@ extension Source {
             src.tryEat(sequence: "atomic_script_run:") {
           return .atomicScriptRun
         }
+
+        throw ParseError.misc("Quantifier '*' must follow operand")
       }
 
       // (_:)

Tests/RegexTests/LexTests.swift

@@ -85,7 +85,12 @@ extension RegexTests {
       "12", base: "U", expectedDigits: 8)
     diagnoseUniScalarOverflow("{123456789}", base: "u")
     diagnoseUniScalarOverflow("{123456789}", base: "x")
-    
+
+    // Test expected group.
+    diagnose(#"(*"#, expecting: .misc("Quantifier '*' must follow operand")) {
+      _ = try $0.lexGroupStart()
+    }
+
     // TODO: want to dummy print out source ranges, etc, test that.
   }