Support classless construct

## 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.
```py
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
```py
from manim import *
```
but I haven't decided on whether I would like to see support of that too.

Start at the existing `manim` command entry point that loads scene classes, then compare that flow with the classless `construct(self: Scene)` example in the issue. Define completion as accepting the proposed construct function while preserving existing class-based scenes; determine separately whether omitting `from manim import *` is in scope.

Written by the indexing model from the issue text.