`AsyncFileWriter` doesn't need to be `Send`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

The `AsyncFileWriter` doesn't need to be `Send` and is therefore too restrictive. For context, I want to implement `AsyncFileWriter` in a WASM context with `js_sys` types. However, all `js_sys` types are `!Send` which makes it impossible to implement.

**Describe the solution you'd like**

Remove the `Send` requirement for `AsyncFileWriter`

**Describe alternatives you've considered**

I can implement the `AsyncFileWriter` using only rust types which involves writing to a buffer and waiting for completion, but I don't particularly want to do that in this context because I'd like to upload while writing to reduce memory usage in the browser.

Locate the `AsyncFileWriter` definition and its `Send` requirement, then inspect existing implementations and tests that exercise the trait. Confirm which implementations and tests depend on `Send`; done means WASM-compatible `js_sys` types can implement the writer without breaking existing behavior.

Written by the indexing model from the issue text.