As a first step towards a simpler Web API, convert `SourceAPI` from
a class to a constant object, under the assumption that `SourceAPI`
isn't a public Ruffle API and as such is safe to be changed.
As a result the different `ruffle-core` users don't need to construct
a new `SourceAPI` instance before calling `PublicAPI.negotiate()`.
Use wasm-bindgen's built-in loader instead of relying on Webpack.
In order to still respect the `publicPath` config, set
`__webpack_public_path__` just before fetching the WebAssembly module.
This allows to no longer declare `.wasm` files as resource assets in
each `webpack.config.js`.
This simplifies the `publicPath` function and makes it useable inside
`fetchRuffle`.
Assuming `publicPaths` isn't used anywhere, this shouldn't be harmless.
relrelb