Support classless construct

[GniLudio]

Description of proposed feature

It would be nice if the manim commands supports construct functions with a surrounding class, similar to how the manimator does it on discord.

from manim import *

def construct(self: Scene) -> None:
    self.add(Circle())

How can the new feature be used?

1. Makes it easier to set up small scenes.
2. Makes it easier to use snippets that were shared on discord.

Additional comments

The manimator also supports omitting the

from manim import *

but I haven't decided on whether I would like to see support of that too.

[behackl]

See #1901 for a proof of concept!

[GniLudio]

See #1901 for a proof of concept!

I can understand how a decorator would be wonderful for many situations, but it doesn't really address the friction when running snippets meant for the manimator.

[behackl]

Ah, yes: I do think it is the better pattern if users have to declare explicitly which functions are intended as scene functions and which aren't. If we had a easier-to-type mechanism available already, the current Discord bot would also not support "plain function" rendering, but would require a (say) decorated function.

We could of course also have the CLI parse a given module to see whether there are suitable plain functions in there that could be used as construct functions. I haven't thought too much about this, initially I'm somewhat worried about false-positives -- but I guess I could be convinced that this is a good idea regardless. No strong opinion, I just like the explicit declaration of something being a function to be used as a construct method.