Change NumberLine's number_scaling based on number length

## Enhancement proposal

`NumberLine` will use the same scaling regardless of the length of the number labels:

```py
class Example(Scene):
def construct(self):
axes = Axes(
x_range=[-2, 2, 0.5],
y_range=[-14, 14, 3.75],
x_length=10,
axis_config={"color": GREEN, "include_numbers": True},
tips=False,
)
self.add(axes)
```


In a previous PR @hydrobeam suggested changing `number_scale_value` based on the length of the numbers so that it doesn't get squashed for longer numbers.

## Additional comments

Start by locating NumberLine and the number_scale_value handling, then reproduce the Axes example from the issue with labels of different lengths. The work is done when NumberLine scaling adapts to label length without squashing longer numbers; add or update focused tests if the existing NumberLine test coverage is found.

Written by the indexing model from the issue text.