Don't allow escaping newlines in single-line literals

We were missing a check for multi-line literals
here.

Author: hamishknight

Sources/SwiftParser/Lexer/Cursor.swift

@@ -1777,7 +1777,7 @@ extension Lexer.Cursor {
             error: error,
             stateTransition: .push(newState: .inStringInterpolationStart(stringLiteralKind: stringLiteralKind))
           )
-        } else if self.isAtEscapedNewline(delimiterLength: delimiterLength) {
+        } else if stringLiteralKind == .multiLine && self.isAtEscapedNewline(delimiterLength: delimiterLength) {
           return Lexer.Result(
             .stringSegment,
             trailingTriviaLexingMode: .escapedNewlineInMultiLineStringLiteral

Tests/SwiftParserTest/ExpressionTests.swift

@@ -1666,6 +1666,21 @@ final class StatementExpressionTests: XCTestCase {
     )
   }
 
+
+  func testUnterminatedString6() {
+    AssertParse(
+      #"""
+      "abc1️⃣\2️⃣
+      (def)3️⃣"4️⃣
+      """#,
+      diagnostics: [
+        DiagnosticSpec(locationMarker: "1️⃣", message: "invalid escape sequence in literal"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: #"expected '"' to end string literal"#),
+        DiagnosticSpec(locationMarker: "3️⃣", message: "consecutive statements on a line must be separated by ';'"),
+        DiagnosticSpec(locationMarker: "4️⃣", message: #"expected '"' to end string literal"#),
+      ]
+    )
+  }
   func testStringLiteralAfterKeyPath() {
     AssertParse(
       #"""