Create an extension

[franmper]

This PR:

• Creates an extension to add a simple cell with a print statement.
• Adds a new Left Panel Sidebar with basic cell information of the active notebook.

Known Issues:

• Even if this has support for different themes selected, it will not work correctly when the theme. I have to add a listener for this using this._themeManager?.themeChanged.connect(this._onThemeChanged, this);