Fix emscripten_websocket_deinitialize()

[strandfield]

Hello,

First of all, thank you for making this great piece of technology.
Running my C++ in a web browser feels overpowered and is really fun.

I was looking at libwebsocket.js trying to understand how it works and figured that emscripten_websocket_deinitialize() likely does not work as it references an undefined variable (WS.sockets).
It seems another user came to the same conclusion (issue #24613).

As far as I can tell, this bug was introduced in commit e1c39bc, when the javascript array of websockets was replaced by an HandleAllocator.
All references to WS.sockets were correctly replaced, save for the ones in emscripten_websocket_deinitialize() ; introducing the bug.

This pull requests consists of 3 commits.
The first one extends the test_websocket_send test to cover the use of emscripten_websocket_deinitialize() and fails with the current code.
The second commit ought to fix the bug and make the test pass.
The last commit is more of a suggestion than anything else, it adds a list() method to HandleAllocator in order to avoid iterating over the allocated array directly.

I ran the following command to test the code and it outputs "OK".

test/runner sockets.test_websocket_send* --browser "C:\Program Files\Google\Chrome\Application\chrome.exe"

There are many test cases I couldn't get to work, even on the main branch ; but that is probably a skill issue on my part.
I hope the command I ran is sufficient.

I also tried to run clang-format on test_websocket_send.c after modifying the file but it produced so many diffs I rolled back the changes. This file doesn't seem to adhere to the style as defined in .clang-format (very long lines, right-aligned pointers) so I left it that way.

I think the following may be used as commit message if the branch is squashed:

Fix emscripten_websocket_deinitialize()

Some references to the javascript array `WS.sockets` were left out when 
the array was replaced by a `HandleAllocator` in e1c39bc59, hence breaking 
the function.

The bug is fixed by iterating over the valid ids in the `HandleAllocator`
and calling `emscripten_websocket_delete()` on them to delete the websockets.

The `test_websocket_send` is extended to cover the function and highlight the 
differences between using  `emscripten_websocket_close()` + `emscripten_websocket_delete()`
or `emscripten_websocket_deinitialize()`.

Fixes #24613

Please let me know if there is anything wrong with the code.
Thank you for your time.

Best regards,
Vincent.