Many people are confused by the fact that LC allows you to put any codec in any format only to get an error when exporting.
Would be nice to warn the user if they try to put for example a H265 inside an MP3 file.
However there are over 100 different formats supported by ffmpeg, and I don't have a list of which codecs ffmpeg supports inside which formats.

Maybe only do it for the most common formats.

See #540

• https://en.wikipedia.org/wiki/Comparison_of_video_container_formats
• https://stackoverflow.com/questions/65263319/ffmpeg-mapping-between-format-and-their-supported-codecs
• https://stackoverflow.com/questions/47276731/how-can-i-determine-if-a-codec-container-combination-is-compatible-with-ffmpeg

Also for subtitles:

• warn the user if they try to put unsupported subtitle inside mp4 warn the user if they try to put unsupported subtitle inside mp4 #700

Also for merging/concatenating files of different codec:

• show warning if codecs are different
• allow converting codecs to a common codec Merge different codec files? #1203