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/test-common-tests.js

@@ -53,6 +53,9 @@ test("toHaveLength", () => {
     expect([1]).toHaveLength(1);
     expect({ length: 1 }).toHaveLength(1);
 
+    expect("a\
+b").toHaveLength(2);
+
     expect(() => {
         expect(1).toHaveLength();
     }).toThrow(ExpectationError);