fix: space_out_submobjects method improved

[GoThrones]

Overview: What does this pull request change?

space_out_submobjects method was scaling the submobjects unevenly from the mobject's center. The submobjects were shifting in x, y directions simultaneously. Also, it was not working for Tex mobjects, but only for Text mobjects. This PR makes the necessary changes so that the submobjects space out only in x direction, and it works for Tex and Text both.

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

When this code is run:

from manim import *

class tryspacingout(Scene):
    def construct(self):
        t1 = Tex("Manim is cool").shift(1.25*UP)
        t2 = Text("Manim is awesome").shift(1.25*DOWN)
        self.play(t1.animate.space_out_submobjects(),
                  t2.animate.space_out_submobjects(factor = 3.5),
                  run_time = 3, 
                  rate_func = there_and_back
                  )

The output is:

But the code in this PR makes this output:

tryspacingout.mp4

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