Add a modernizr check for WebAssembly support

[dbkr]

We don't work at all without this now and currently fail in terrible ways. This will cause us to display the 'unsupported browser' screen if we don't have wasm.

Also comment the three different types of error page.

Playwright test coming for this in react-sdk.

Fixes #27735

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[t3chguy]

I don't think this is sufficient, I believe rust crypto needs the modern API for loading it also. e.g. https://developer.mozilla.org/en-US/docs/WebAssembly/JavaScript_interface/instantiateStreaming_static rather than https://developer.mozilla.org/en-US/docs/WebAssembly/JavaScript_interface/instantiate_static (FF 58 vs FF 52)

[dbkr]

So it actually doesn't, because it seems the lowest-common-denominator way was the only reliable way of loading it portably, so it actually uses WebAssembly.instantiate() in practice. We could certainly try loading a simple module using that: that would cover CSP issues too, but we'd be assuming more details of how the library works, which could change without us knowing so we could end up saying it won't work even when it actually would.

[richvdh]

What @dbkr says is true: we wanted to package the wasm blob in a way that it could be loaded by applications using the js-sdk without a whole lot of custom packaging. Doing that, whilst allowing EW to stream it, seemed approximately impossible at the time, mostly because everything in the JS ecosystem is awful. Hence, we use WebAssembly.instantiate rather than WebAssembly.instantiateStreaming.

(It may be worth revisiting that situation now that we have Webpack 5 and the JS-SDK is built as ES modules, but I digress.)

What we do do is use WebAssembly.instantiate rather than calling the WebAssembly.Module constructor, which loads the wasm synchronously. But both methods seems to have landed in the same version of FF, so I don't think it makes any difference.