Implements a php.runStream() method that returns a StreamedPHPResponse instance.
It exposes stdout and stderr as ReadableStreams, allowing the caller to interact
with partial output data. Before this PR, we only had php.run() that buffered 
stdout and stderr data and returned it all at once after the PHP code was fully
executed.
## Implementation
We register three FS devices at:
* /internal/stdout
* /internal/stderr
* /internal/headers
They are private to every PHP instance and are never shared with other runtimes.
Then, in JavaScript, whenever a chunk of data is written to either of these devices, we propagate
it to consumer via a callback, e.g. `PHPWASM.onStdout(chunk)`.
Users of the `PHP` class never have to interact with these devices or callbacks directly.
The PHP class creates the relevant ReadableStreams and pushes the data through them
– see php.#executeWithErrorHandling() for details.
#### Why not use Emscripten's stdout and stderr?
Emscripten's native stdout and stderr devices stop processing data when they encounter
the first null byte. However, null bytes are common when dealing with binary data.
### Backwards Compatibility
php.run() continues to work. It creates a streamed response under the hood and buffers
the streamed output before returning a buffered `PHPResponse` object.
## Remaining work
* Add streaming-specific tests
## Follow-up work
* Stream the response bytes in the web/service worker
Remove old PHP CLI bindings