WasmFS: OPFS AccessHandles backend

[tlively]

We should write a new WasmFS backend using OPFS AccessHandles to provide efficient access to the native file system.

Depends on #15946.

[tlively]

This was started in #16813. The backend creates an exclusive AccessHandle for each opened file, so each file can only be opened once at a time. Improving that is tracked in #17121.

We also need to make the error handling in the opfs backend more robust with tests for the different things that could go wrong.

[tlively]

The error handling issue has been fixed and the OPFS backend is generally usable now.

[rulkens]

Is there any documentation on this? I've looked in the emscripten docs but I can't find any info about it (https://emscripten.org/docs/api_reference/Filesystem-API.html#file-system-api). I'd love to use it for my custom PNG encoder wasm implementation.

[hangxingliu]

@rulkens coincidence, i am also looking for it for a better data persistence process. I found that there is a good example c source code, which can be used as a documentation at https://github.com/emscripten-core/emscripten/blob/main/test/wasmfs/wasmfs_opfs.c