Added a \overset as a special string

[EbbDrop]

Motivation

Makes something like Tex("\overset", "{def A}", "{=}") work.

Proposed changes

Added "\\overset" -> "\\overset{\\quad}{\\quad}" and documented why \frac and friends do not work (yet).

Test

Code:

  color = {"isolate": ["{A}", "{B}"], "tex_to_color_map": {"{A}": GREEN, "{B}": BLUE}}
  t = Tex(r"a{B}c", r"\overset", "{A}{=} a{B}c", **color)
  self.play(Write(t))

Result: