Rewrite MathTex to make it more robust regarding splitting

[henrikmidtiby]

Overview: What does this pull request change?

This PR changes a large part of the implementation of the MathTex class.
The reason is to deal with the issues related to splitting the tex string and then try to match the generated objects with parts of the original tex string.
The idea for the PR is based on this discussion on discord.
https://discord.com/channels/581738731934056449/1406977902788218892

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

Links to added or changed documentation pages

Further Information and Comments

This PR is an extension of #4473 "Maintain groups when importing svg files"

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

[henrikmidtiby]

And here are some example code and the generated output.

class ExampleScene8(Scene):
    def construct(self):
        formula = MathTex(
            r"P(X=k) =",
            r"\binom{12}{k}",
            r"0.5^{k}",
            r"(1-0.5)^{12-k}",
            substrings_to_isolate=["k", "1", "12", "0.5"],
        ).scale(1.3)

        formula.set_color_by_tex("k", GREEN)
        formula.set_color_by_tex("12", RED)
        formula.set_color_by_tex("1", YELLOW)
        formula.set_color_by_tex("0.5", BLUE_D)
        self.add(formula)

class ExampleScene11(Scene):
    def construct(self):
        t2cm = {"n": RED, "1": GREEN, "x": YELLOW}
        eq = MathTex(r"\sum_{1}^{n} x", tex_to_color_map=t2cm).scale(1.3)

        self.add(eq)