Implement Manim cell preview & tidy up entry points

[Splines]

Warning

This PR also includes all changes of #7 as it is a follow-up. #7 must be merged beforehand.

Closes #3.

• This is a follow-up of Init Manim Cell detection #7 where the Manim Cell detection mechanism was initialized. Here, we implement the actual previewCode() method.
• Furthermore, we implement a new command to preview selected text. However, note that I don't think that this is what we actually want (see review comment below).
• The extension.ts is cleaned up by outsourcing into individual functions with docstrings.
• We also implement the first user setting (clipboard timeout).

Parts of this PR were inspired by #5 by VladimirFokow.

showcase.manim.cell.preview.mp4

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When squashing and merging this PR: in the body of the generated message, could you remove all lines up to (including) "Fix spelling mistake"? The first new commit of this branch is "Document all failed attempts to capture terminal output". It's just that GitHub somehow doesn't realize that the prior commits are already included in the base branch.

[bhoov]

Cell execution works as expected. This is an awesome contribution.