new WASI RID

[pavelsavara]

To complement adding new RID for compiling Mono to WebAssembly System Interface platform.

See also dotnet/runtime#78376 and dotnet/runtime#77780

[jkotas]

WASI -> Wasi to make it match dotnet/runtime#78389 ?

[pavelsavara]

WASI -> Wasi to make it match dotnet/runtime#78389 ?

That was after API people feedback.
There was feedback in opposite direction from @vargaz here dotnet/runtime#78376 (comment) so I made it uppercase WASI in that PR in most places. Should we reconsider ?

I don't have strong preference either way.

[akoeplinger]

I'd use Wasi for the TestPlatforms enum for consistency, we can use WASI in the non-.NET build/project files.

[radical]

cc @lewing