Reduce the time for CLI startup

Description of the proposed feature

Currently, when running manim executable or module using (py -m manim), the whole module is imported which is large and takes a lot of time. I propose to move all the CLI handling stuff (ie manim.cli) to a new module called manim_cli which would not import manim until the command line arguments are parsed. By doing so, we can reduce the startup time.

How can the new feature be used?

Reduce the time to load manim command. If someone just wanted to see the help this would reduce the time they wait because manim wouldn't be imported in that case.

Additional comments

We would need to move the main module to src structure for this to work.

src \
    - manim
    - manim_cli

Start by inspecting the current manim executable and module entry points, especially manim.cli, and measure the existing startup time. Review how the package is laid out under src and determine how a separate manim_cli module could parse arguments before importing manim. Done means the CLI starts faster, including help output, while existing command behavior remains available.