Support moving the tasks framework to use the integrated terminal

[Tyriar]

It makes sense to reuse the work of the integrated terminal in the tasks framework to leverage things like:

• Handling of ANSI control characters (colors, cursor movement)
• Multiple tasks
• Allowing input

Here are the blockers to allow this to happen:

• Expose a terminal API option to pause when the shell is exited #15583 Expose a terminal API option to pause when the shell is exited
• Terminal should throw an event when a terminal has completed (and is waiting to close) #18376 Terminal should throw an event when a terminal has completed (and is waiting to close)
• Allow terminals to be reused #18377 Allow terminals to be reused

No longer needed:

• Allow extensions to access all terminals via API #13267 Allow extensions to access all terminals via API (deferred as it's not a mandatory requirement and poses security/performance obstacles)
• Allowing richer integration with integrated terminal (co-processes) #13337 Allowing richer integration with integrated terminal (co-processes) (internal API exposed for now 7900f25)~~

After this is done tasks could also be moved to an extension.

[Tyriar]

For the problem watcher I'm thinking that something similar to an iterm2 trigger with a callback and then some way of grabbing stdout would be sufficient (feature request: #13337).