Split Theia terminal widget logic

[evidolob]

Description

Sub task of #11442
Split terminal widget logic:

• terminal widget part (without any connection logic).
• terminal connection logic.

This split means: define common interface and default implementation for connection part to the separated Theia package: terminal. Xterm.js specific implementation of the terminal widget move to the separated package xterm.js-terminal

[che-bot]

Issues go stale after 180 days of inactivity. lifecycle/stale issues rot after an additional 7 days of inactivity and eventually close.

Mark the issue as fresh with /remove-lifecycle stale in a new comment.

If this issue is safe to close now please do so.

Moderators: Add lifecycle/frozen label to avoid stale mode.