Added ~50 extensions mostly related to macOS

[beninato8]

I went through all the extensions on my computer and added each one that I recognized. I also reorganized and added some categories (for example, creating a macOS category and moving XCode to be a subcategory in it). Each commit message should give a brief explanation of what I added, but I would be happy to clarify any of them.

I found all the extensions by running

for x in **/*; do if [ -f $x ]; then echo $(ext $x); fi; done | uniq | gsort

where ext is aliased to

getExtension () {
    fullname=$(basename -- "$1")
    extension="${fullname##*.}"
    echo $extension
}

[rpdelaney]

This is fantastic, thank you!