Remove Null host

[Marco-Farruggio]

Remove the null host, add a compilation error on unsupported targets, and updated the documentation. To me expecting cpal to compile on a target with no audio capabilities is like expecting a GUI library to compile on a target with no display server, which should really be a build error. This also means its less clutter in the codebase which is always a plus.

Closes #1366

[Marco-Farruggio]

Related: I think #571 can be closed because WASI doesn't support audio, and the proposed wasi-media isn't near starting

[LastExceed]

I have closed #571 . Think you can get CI to pass?

[Marco-Farruggio]

Sorry, I only just saw this notification, and yes, I'll put some time aside tomorrow to get this ready to merge :)

[Marco-Farruggio]

What do you think is a better idea for when there is no cpal host available for a certain compilation target and default_host() is called?:

1. return the CustomHost, if its present, else emit a compile error stating there is no supported host, and maybe a useful message about the documentation and implementing a custom host.

2. always emits a compilation_error!, even if CustomHost is present, in which case the user must explicitly use CustomHost (via host_from_id).

[Marco-Farruggio]

Well it actually can't currently return CustomHost as CustomHost::new() is hardcoded to return an error, as it was expectedly to never be called

[LastExceed]

Just do whatever is easier right now, we can always change it later

[Marco-Farruggio]

Why do we have CI checks for WASI when it's unsupported? They won't pass anymore as I emit a compilation error

[LastExceed]

No idea. Just remove them

[Marco-Farruggio]

I think the WASM checks were actually all emitting false positives, as after removing the NullHost, it seems like that was all that was keeping them passing, I'm going to remove the WASI CIs now and see what I can do about the others (I haven't ever meddled with CIs before)

[Marco-Farruggio]

Welp one removed null host later, a lot of comments, doc rewriting, and going down a CI rabbit hole for me, I think it's ready for merging 😅