[Browser] Support file uploads

[adamziel]

At the moment, uploading files e.g. in media library isn't supported by the PHP request handler. Here's one way to solve it:

• Pass the uploaded data blob from the service worker to the WASM worker
• Create a temporary file in the WASM worker
• Setup the correct $_FILES entries in the WASM worker

[adamziel]

Solved by #48. Follow-up work in #49 and #50