LibJS: Implement rules for duplicate function parameters

- A regular function can have duplicate parameters except in strict mode
  or if its parameter list is not "simple" (has a default or rest
  parameter)
- An arrow function can never have duplicate parameters

Compared to other engines I opted for more useful syntax error messages
than a generic "duplicate parameter name not allowed in this context":

    "use strict"; function test(foo, foo) {}
                                     ^
    Uncaught exception: [SyntaxError]: Duplicate parameter 'foo' not allowed in strict mode (line: 1, column: 34)

    function test(foo, foo = 1) {}
                       ^
    Uncaught exception: [SyntaxError]: Duplicate parameter 'foo' not allowed in function with default parameter (line: 1, column: 20)

    function test(foo, ...foo) {}
                          ^
    Uncaught exception: [SyntaxError]: Duplicate parameter 'foo' not allowed in function with rest parameter (line: 1, column: 23)

    (foo, foo) => {}
          ^
    Uncaught exception: [SyntaxError]: Duplicate parameter 'foo' not allowed in arrow function (line: 1, column: 7)

Libraries/LibJS/Tests/builtins/Array/Array.prototype.forEach.js

@@ -51,7 +51,7 @@ describe("normal behavior", () => {
     test("callback receives array", () => {
         var callbackCalled = 0;
         var a = [1, 2, 3];
-        a.forEach((_, _, array) => {
+        a.forEach((_, __, array) => {
             callbackCalled++;
             expect(a).toEqual(array);
             a.push("test");