Add a tree navigation pane showing Manim cells (outline)

[Splines]

With the Tree View API, we could show an outline of the Manim Cells of the currently open file, just like there exists outlines for languages where we can see what methods and properties a class defines etc. This can greatly easy navigation in big files.

• Location: In the explorer view as additional pane with the name Manim Cells. This is just the default position, users can then drag the pane around like they want.
• Item string: the top-comment of the Manim Cell, i.e. the part after ##
• Items are grouped/folded by their classes in which they are defined (I don't think an additional function layer is really necessary)
• On Click: jump to the respective Manim Cell.
• Bonus: Show a preview button (e.g. play button) right next to the item in the tree view.

See for example how an outline looks like

[bhoov]

This would be amazing. Definitely not necessary for v0.1.0 though