Run the DevFile Commands by calling the che-terminal extension's API

[azatsarynnyy]

Signed-off-by: Artem Zatsarynnyi [email protected]

What does this PR do?

1. che-terminal extension exposes its API to get a vscode.Pseudoterminal that is passed to a Tasks's vscode.CustomExecution.
2. Switches the che-commands extension to use che-terminal extension API when running the Devfile Commands.

This is step 2/3 in migrating from the current implementation of the machine-exec-based terminal, that replaces the built-in VS Code terminal, to the dedicated extension.
1/3 was introduced in #122.

What issues does this PR fix or reference?

Related to:

• Implement a VS Code extension for opening the container terminals eclipse-che/che#21673
• PowerShell VS Code extension failed to start eclipse-che/che#21626

How to test this PR?

1. Start a DevWorkspace with the test project https://github.com/azatsarynnyy/java-spring-petclinic/tree/commands-ext
2. Try to run a Che Task. E.g.: Terminal > Run Task... > che.
3. The task should be executed as expected, without any visible changes. Under the hood, the task is executed by calling the che-terminal extension's API. It can be verified by checking the Che Terminal output channel (View > Output).