[JSC] merge Iterator.prototype.sliding into Iterator.prototype.windows

https://bugs.webkit.org/show_bug.cgi?id=299380

Reviewed by Yusuke Suzuki.

Rather than having two different methods that are almost identical, merge them into one with an argument to control the behavior.
- `iter.windows(num)` and `iter.windows(num, "only-full")` behave the same as before.
- `iter.windows(num, "allow-partial")` behaves the same as `iter.sliding(num)`.

Proposal: <https://github.com/tc39/proposal-iterator-chunking>
Spec: <https://tc39.es/proposal-iterator-chunking/>

* Source/JavaScriptCore/builtins/JSIteratorPrototype.js:
(windows):
(sliding): Deleted.
* Source/JavaScriptCore/runtime/JSIteratorPrototype.cpp:
(JSC::JSIteratorPrototype::finishCreation):

* JSTests/stress/iterator-prototype-sliding.js: Removed.
* JSTests/stress/iterator-prototype-windows.js:

* JSTests/stress/iterator-prototype-chunks.js:

Drive-by: tweak wording of error messages for clarity.
Canonical link: https://commits.webkit.org/300551@main

Author: dcrousso

JSTests/stress/iterator-prototype-chunks.js

@@ -151,7 +151,7 @@ function shouldThrow(fn, error, message) {
     for (const invalidChunkSize of invalidChunkSizes) {
         shouldThrow(function () {
             Iterator.prototype.chunks.call(validIter, invalidChunkSize);
-        }, RangeError, "Iterator.prototype.chunks requires that argument not be NaN.");
+        }, RangeError, "Iterator.prototype.chunks requires that first argument not be NaN.");
     }
 }
 
@@ -166,6 +166,6 @@ function shouldThrow(fn, error, message) {
     for (const invalidChunkSize of invalidChunkSizes) {
         shouldThrow(function () {
             Iterator.prototype.chunks.call(validIter, invalidChunkSize);
-        }, RangeError, "Iterator.prototype.chunks requires that argument be between 1 and 2**32 - 1.");
+        }, RangeError, "Iterator.prototype.chunks requires that first argument be between 1 and 2**32 - 1.");
     }
 }