fix: return value of TexTemplate.add_to... (#3343)

Unlike similar methods on other objects .add_to_preamble and
.add_to_document do not return the TexTemplate object itself.
This does not allow for stacking these methods.

Just added the `return self` for both methods

Co-authored-by: Uwe Zimmermann <C:\Users\uwezi\AppData\Roaming\The Bat! Pwd>

Author: uwezi

manim/utils/tex.py

@@ -139,6 +139,7 @@ def add_to_preamble(self, txt: str, prepend: bool = False):
         else:
             self.preamble += "\n" + txt
         self._rebuild()
+        return self
 
     def add_to_document(self, txt: str):
         """Adds txt to the TeX template just after \\begin{document}, e.g. ``\\boldmath``
@@ -150,6 +151,7 @@ def add_to_document(self, txt: str):
         """
         self.post_doc_commands += "\n" + txt + "\n"
         self._rebuild()
+        return self
 
     def get_texcode_for_expression(self, expression: str):
         """Inserts expression verbatim into TeX template.