Enable Yew Runtime for wasm32-wasi

[futursolo]

Tokio has just landed support for wasm32-wasi. However, wasi misses some essential features for Yew Runtime to become useful under wasi.

wasi can be used for:

• Client-Side Rendering (wasi in Browser)
• Server-Side Rendering (wasi in JavaScript-capable WebAssembly Runtime & Standalone WebAssembly Runtime)
• Static-Site Generation (wasi in JavaScript-capable WebAssembly Runtime & Standalone WebAssembly Runtime)

This issue tracks the tokio features required to land wasi support for Yew Runtime:

• [ ] wasi target currently lacks networking capability in general. (needed for CSR and SSR.) This is primarily due to sockets under wasi is not standardised. There is an alternative proposal for an HTTP interface.
• [ ] tokio::fs for wasi. (needed for SSG and potentially SSR.)

There are some other features that might be desirable before we start to implement the target:

• [ ] multi-threading support so multiple CPU cores can be utilised.