Support compiling `sway-core` and `forc-pkg` to WASM?

This is a bit of a moonshot issue and by no means a priority, but I've been meaning to open it for a while and would likely enable some pretty cool stuff (e.g. in-browser compilation, deploying from the browser, etc).

Some steps that come to mind in enabling this:

- Review all `I/O` throughout `sway-core` and `forc-pkg` and consider browser-friendly alternatives. This would likely require support for handling in-memory representation of sway files, seeing if it's possible to compile git/ipfs deps to WASM, etc.
- Add a `wasm32-unknown-unknown` build job to CI. Add one package at a time to avoid having to overhaul the whole repo in one PR.

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On a related note: cc @JoshuaBatty is doing something like this a requirement to support Sway in vscode.dev? Or they support native plugins somehow?

Start by reviewing all I/O in sway-core and forc-pkg, including how Sway files and git/IPFS dependencies are represented. Assess browser-friendly in-memory alternatives and add a wasm32-unknown-unknown CI build one package at a time. Done means the targeted packages compile for WASM and CI covers them.

Written by the indexing model from the issue text.