Support for wasm32-wasi target

[bkolobara]

I ran into an issue while trying to compile plotters to WebAssembly.

Currently any wasm32-*-* target (e.g. wasm32-unknown-unknown, wasm32-wasi, wasm32-...) is considered to be running inside the browsers and uses wasm-bindgen` as a dependency.

But this is not always the case. In my case I'm trying to run it on the backend inside an application compiled to wasm32-wasi on Wasmtime and Wasmer. The issue is that wasm-bindgen doesn't work on this platforms because they are not browsers and the required wasm-bindgen imports are not satisfied.

Would plotters still work if we just treated wasm32-wasi as a non-wasm backend?

[tqwewe]

@38 I believe this issue should be reopened.

When trying to work with plotters on wasm32-wasi target, I get some errors:

error[E0432]: unresolved import `font_kit::source::SystemSource`

If I try adding font-kit with feature source-fontconfig-default, I get a different error:

error[E0432]: unresolved imports `libloading::Library`, `libloading::Symbol`

[tqwewe]

Nevermind, I got it working by removing the ttf and bitmap features. 👍