Allow empty comments

`expectQuoted` expects non-empty contents, which
doesn't apply to comments.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -630,10 +630,10 @@ extension Source {
   mutating func lexComment(context: ParsingContext) throws -> AST.Trivia? {
     let trivia: Located<String>? = try recordLoc { src in
       if src.tryEat(sequence: "(?#") {
-        return try src.expectQuoted(endingWith: ")").value
+        return try src.lexUntil(eating: ")").value
       }
       if context.experimentalComments, src.tryEat(sequence: "/*") {
-        return try src.expectQuoted(endingWith: "*/").value
+        return try src.lexUntil(eating: "*/").value
       }
       if context.endOfLineComments, src.tryEat("#") {
         // Try eat until we either exhaust the input, or hit a newline. Note

Tests/RegexTests/ParseTests.swift

@@ -1648,6 +1648,9 @@ extension RegexTests {
     parseTest("[(?#abc)]", charClass("(", "?", "#", "a", "b", "c", ")"))
     parseTest("# abc", concat("#", " ", "a", "b", "c"))
 
+    parseTest("(?#)", empty())
+    parseTest("/**/", empty(), syntax: .experimental)
+
     // MARK: Matching option changing
 
     parseTest(