[Client] Enable calling playground.runStream() and .cli() from the client

[adamziel]

Motivation for the change, related issues

Makes playground.cli() and playground.runStream() available for Playground integrators.

Without this PR, these methods can only be called from the same JavaScript process where the PHP is running. If you try to call it over Comlink, you get an error TypeError: Unserializable return value as Comlink doesn't know how to transfer a StreamedPHPResponse object over to another worker..

With this PR, you can call both these methods on a PHP Client and Comlink will correctly transfer the StreamedPHPResponse object.

This unlocks exposing a XTerm.js widget with WP-CLI and Composer commands.

Implementation details

Implements a new transfer handle that uses a few lengthy utility methods to convert between promises and streams and MessagePort instances. On most browsers, it uses the transferable property of a ReadableStream. On Safari, it falls back to MessagePorts.

Testing Instructions (or ideally a Blueprint)

CI – this PR ships a new E2E test.

Also, try this in devtools:

await playground.writeFile('/tmp/script.php', "<?php echo 'hi!'; ");
const r = await playground.cli(['php', '/tmp/script.php']);
await r.stdoutText

You should se "hi!".

cc @mho22 @brandonpayton