--extension <extension>extension, the file extension that we should build
The extension flag is used to tell square which type of build you want to output. As square is primarily build for front-end development it currently supports the following output extensions:
cssCascading Style Sheets.
The extension flag is only used for finding files that either have that
extension or compile code to that extension. For example if you use
--extension css it will compile a build with all the
files that you specified in your bundle.
square ---bundle . --extension css