Shell integration: Use command tracking as alternative child process tracking

[Tyriar]

Use command tracking to save running a process for whether the shell is dirty, similar to cwd tracking this will lead to more responsive UI and less work happening on update. https://github.com/microsoft/vscode/blob/a1bf964306d30a9bbccb404b1b653e5a02fe3052/src/vs/platform/terminal/node/childProcessMonitor.ts

[meganrogge]

Blocked by supporting subshells

[Tyriar]

Created the subshell issue #141343, instead of relying on sub-shell support, we could just say when the shell is not in the prompt, the terminal's busy. This is arguably better than the current child process support anyway as the terminal is doing something. Sub-shells are a problem as it would always be busy, but that's a problem that could be handled with #131343.

Need to way the pros/cons for both ways.

[Tyriar]

Ideas:

• Move child process tracking into a capability which uses onFirstListenerAdd and onLastListenerRemove to only turn that mechanism on when needed
• If the command detection capability is available, prefer that and expose a new property/event for when the cursor is in the prompt.

This should make the dirty indicator much more responsive as well.