expose IBufferSet API

[JavaCS3]

See #2694

[Tyriar]

I was thinking we're completely replace buffer, making buffer a namespace and moving the actual IBuffer object lower:

class Terminal {
  readonly buffer: IBufferNamespace;
}

interface IBufferNamespace {
  readonly normal: IBuffer;
  readonly alternate: IBuffer;
  readonly active: IBuffer;
  // Change as that's the word used in other APIs
  onBufferChange: IEvent<IBuffer>;
}

interface IBuffer {
  type: 'normal' | 'alternate';
}

This would need changes in addons but buffer is experimental so that's fine.

[JavaCS3]

@Tyriar But there're a lot of test cases using buffer API.

I would suggest adding a new buffers API and deprecate the old API, that will make the transition smoother

[Tyriar]

@JavaCS3 it's primarily just internal things though, most of those would probably get fixed by doing a project-wide find and replaces, for example "term.buffer.x" -> "term.buffers.active.x"

[JavaCS3]

@Tyriar Updated

[JavaCS3]

@Tyriar Updated, Looks like there're a lot of pending PRs. Does xterm.js have any schedule to clean them up?

[Tyriar]

@JavaCS3 I've been a bit busy with other things in VS Code, this week has been catching up on this stuff and hopefully getting a release out.