LibJS: Fix "use strict" directive false positives

By having the "is this a use strict directive?" logic in
parse_string_literal() we would apply it to *any* string literal, which
is incorrect and would lead to false positives - e.g.:

    "use strict" + 1
    `"use strict"`
    "\123"; ({"use strict": ...})

Relevant part from the spec which is now implemented properly:

[...] and where each ExpressionStatement in the sequence consists
entirely of a StringLiteral token [...]

I also got rid of UseStrictDirectiveState which is not needed anymore.

Fixes #3903.

Libraries/LibJS/Tests/use-strict-directive.js

@@ -45,4 +45,16 @@ test("invalid 'use strict; directive", () => {
             return isStrictMode();
         })()
     ).toBeFalse();
+    expect(
+        (() => {
+            `"use strict"`;
+            return isStrictMode();
+        })()
+    ).toBeFalse();
+    expect(
+        (() => {
+            "use strict" + 1;
+            return isStrictMode();
+        })()
+    ).toBeFalse();
 });