Added missing default_numbers_to_display() in NumberLine Class

[zoya5636]

Overview: What does this pull request change?

I added a default_numbers_to_display function in the NumberLine class.

Motivation and Explanation: Why and how do your changes improve the library?

To fix issue #2473 - default_numbers_to_display is currently being called by get_number_mobjects in NumberLine without being defined.

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[JasonGrace2282]

Hello, thanks for your interest in contributing to Manim :)

When I try to test out the code, I get an error about op not existing, and fixing that I get an attribute error about there being no attribute tick_frequency on NumberLine.
For clarity, here's the test scene I used (straight from the issue mentioned)

class NumberlineNumberMobjects(Scene):
    def construct(self):
        line = NumberLine(include_numbers=True)
        numbers = line.get_number_mobjects()
        self.add(line)

I'll mark this PR as a draft until it's ready :)
Feel free to open it when you feel like it's working again.