Blocking write API when in workers

[sgenoud]

C code (as well as Rust code) tends to expect file APIs to be synchronous. Wasm integrations of some libraries that need disk access can be simpler when a blocking API is available.

It would be nice to offer sync versions of the write operations (writeSync, flushSync and closeSync) on the FileSystemWriter interface when in a worker (blocking APIs are always limited to workers).

There is an equivalent for reading data, the FileReaderSync interface.

[a-sully]

Looks like this issue got lost in the shuffle, but this behavior is now available via Access Handle within the Origin Private File System. See #344