Terminal API: On ready event

[Tyriar]

I've heard from several extension authors that they want to know when a terminal is ready, this has historically been difficult to detect and I didn't want to commit to an API but I think something like the below would cover almost all these cases and it could not in the docs that it's not 100%.

// name could do with some work?
onDidReadyTerminal: Event<Terminal>;

This would be based on a heuristic for now, firing when:

• Data has been written to the terminal, but no data within the last 250(?)ms
• A maximum of 3(?)s has elapsed

We could then refine when this fires at a later point if we find a good way to detect this.

[connor4312]

I think this duplicates #127717

[Tyriar]

Not quite, on ready was meant to mean the terminal has finished initializing where we would use insights like from shell integration to know the prompt was ready to go. But there seems to be little interest in it.