Implement a VS Code extension for opening the container terminals

[azatsarynnyy]

Is your task related to a problem? Please describe

Currently, in Che-Code, we have implemented support for opening the terminals into the DevWorkspace containers. On top of that functionality, we have implemented support for running the Devfile Commands in the DevWorkspace containers. These features work well.

Later, we discovered that replacing a VS Code's built-in terminal infrastructure with our machine-exec-based one requires a lot of effort maintaining it in order to:

• ensure the latest VS Code's terminal/task-related features work in Che-Code in the same way as in VS Code (e.g.: Enabling the extended Shell integration breaks the auto-opened terminal in Che-Code #21616)
• ensure the extensions work without any problems with the machine-exec under the hood (e.g.: PowerShell VS Code extension failed to start #21626 or It's not possible to run a Task in Che-Code if it's not che type #21466)

Describe the solution you'd like

We can keep the VS Code's built-in terminal support as it is in the upstream and don't touch it. All the Che-specific functionality can be provided within a dedicated extension(s).
There are a couple of extension APIs in VS Code that should allow us to contribute all the required functionality:

• Pseudoterminal API allows contributing a custom implementation of the PTY device (it's a machine-exec-based in our case)
• CustomExecution API allows executing the Devfile Commands (VS Code Tasks) on the custom pseudoterminal

Describe alternatives you've considered

No response

Additional context

See the result of the investigation in #21626 (comment)

[azatsarynnyy]

upd:
Opening the terminals into DevWorkspace containers with the VS Code Pseudoterminal Extension API works well.
I'm reworking the way how we run the Devfile Commands to use the API of the new extension and VS Code CustomExecution Extension API.

[azatsarynnyy]

The new che-terminal VS Code extension is contributed in the PR che-incubator/che-code#122.
Now, a terminal to a DevWorkspace container is opened using the Pseudoterminal VS Code API.

The existing che-commands extension was reworked in the PR che-incubator/che-code#123.
Now, a Devfile Command is run by calling the che-terminal extension's API using the CustomExecution VS Code API.

Both these steps allowed to return back the built-in VS Code terminal (che-incubator/che-code#124) which enables running all types of VS Code Tasks, but not only the Tasks of che type #21466.
Also, these steps remove all the obstacles to proper work of the PowerShell VS Code extension #21626.