LibJS: Allow invalid string in tagged template literals

Since tagged template literals can inspect the raw string it is not a
syntax error to have invalid escapes. However the cooked value should be
`undefined`.
We accomplish this by tracking whether parse_string_literal
fails and then using a NullLiteral (since UndefinedLiteral is not a
thing) and finally converting null in tagged template execution to
undefined.

Userland/Libraries/LibJS/Tests/tagged-template-literals.js

@@ -105,4 +105,29 @@ describe("tagged template literal functionality", () => {
         expect(raw[1]).toHaveLength(5);
         expect(raw[1]).toBe("\\nbar");
     });
+
+    test("invalid escapes give undefined cooked values but can be accesed in raw form", () => {
+        let calls = 0;
+        let lastValue = null;
+        function noCookedButRaw(values) {
+            ++calls;
+            expect(values).not.toBeNull();
+            expect(values.raw).toHaveLength(1);
+            expect(values.raw[0].length).toBeGreaterThan(0);
+            expect(values.raw[0].charAt(0)).toBe("\\");
+            expect(values[0]).toBeUndefined();
+            lastValue = values.raw[0];
+        }
+        noCookedButRaw`\u`;
+        expect(calls).toBe(1);
+        expect(lastValue).toBe("\\u");
+
+        noCookedButRaw`\01`;
+        expect(calls).toBe(2);
+        expect(lastValue).toBe("\\01");
+
+        noCookedButRaw`\u{10FFFFF}`;
+        expect(calls).toBe(3);
+        expect(lastValue).toBe("\\u{10FFFFF}");
+    });
 });

Userland/Libraries/LibJS/Tests/template-literals.js

@@ -63,3 +63,9 @@ test("line continuation in literals (not characters)", () => {
 test("reference error from expressions", () => {
     expect(() => `${b}`).toThrowWithMessage(ReferenceError, "'b' is not defined");
 });
+
+test("invalid escapes should give syntax error", () => {
+    expect("`\\u`").not.toEval();
+    expect("`\\01`").not.toEval();
+    expect("`\\u{10FFFFF}`").not.toEval();
+});