* chat: remove `waitForReady`

- Now that edit sessions are sync, we don't need to wait for them (just
  guard edits) as in https://github.com/microsoft/vscode-copilot/issues/16060


- I noticed `widget.onDidClear()` listeners were actually async and we
  do need to wait for them before we can safely make new input. Revised
  this to a delegate pattern.
- Wait for a view model when sending chat. Also, improved Event.toPromise
  so it can safely be reused -- it now cleans up its own disposable from
  the disposable store or array after it settles.

Resolves #247484

* Update src/vs/base/common/event.ts

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>