[ext-api] provide Pseudoterminal.onDidChangeName event

[akosyakov]

Current it is not possible to change the name of the pseudo terminal implemented by the extension.

[Tyriar]

Makes sense, if a PR for this comes in for this I can bring it to the API sync. Seems like a good improvement so I don't see why it wouldn't be able to get merged in.

[habibkarim]

I can take this if nobody has already

[Tyriar]

@hbibkrim that would be great 👍

[habibkarim]

@Tyriar Should this go under proposed API?

[Tyriar]

@hbibkrim yes