Add WebTransport webidl to unstable

[darioalessandro]

Added WebTransport webidl definitions to the unstable features.

[darioalessandro]

@Liamolucko let me know the next steps 😄

[Liamolucko]

Looks good! I just left one comment about the URL of the WebIDL file being wrong.

I would have asked you to add [Throws] to any functions that can throw, which tells the code generator the function should return a Result, but it turns out that nothing in WebTransport can actually throw anyway, since everything's async and results in a promise rejection or similar instead.

[darioalessandro]

@Liamolucko change applied, thanks for your review 😄