Configureable paths for FFmpeg/LaTeX

[naveen521kk]

Description of proposed feature

Currently, manim looks in $PATH for finding FFmpeg or LaTeX executables and invokes them. Sometimes a wrong executable is executed and people complain something isn't working and ask for help. With this feature, if the paths for FFmpeg/LaTeX aren't set in the config file, manim should interactively ask for whether it can use the executables it finds in $PATH or ask them to enter the path of the executable.

How can the new feature be used?

One can set ffmpeg_path in config file like

[CLI]
ffmpeg_path = "C:\ffmpeg\ffmpeg.exe"

and Manim will always execute the config.ffmpeg_path instead of finding it from $PATH.

Something similar should exist for LaTeX also

[CLI]
latex_path = "C:\texlive\2021\bin\win32\latex.exe"

and Manim would do the same. The difference from FFmpeg is for other executable and engines (like dvisvgm), it should look for Path(config.latex_path).parent / '<executable_name>'. This should avoid the issue where people use tinytex but dvisvgm is installed using apt (a very common issue I have seen in the installation-help channel).