LibJS: Parse line continuations in string literals properly

Newlines after line continuation were inserted into the string 
literals. This patch makes the parser ignore the newlines after \ and
also makes it so that "use strict" containing a line continuation is 
not a valid "use strict".

Libraries/LibJS/Tests/template-literals.js

@@ -53,6 +53,13 @@ test("newline literals (not characters)", () => {
     ).toBe("foo\n    bar");
 });
 
+test("line continuation in literals (not characters)", () => {
+    expect(
+        `foo\
+    bar`
+    ).toBe("foo    bar");
+});
+
 test("reference error from expressions", () => {
     expect(() => `${b}`).toThrowWithMessage(ReferenceError, "'b' is not defined");
 });