A CustomFunction<'sync'> must return synchronously, but
in return it can be passed to compile and compileString in
addition to compileAsync and compileStringAsync.
A CustomFunction<'async'> may either return synchronously or
asynchronously, but it can only be used with compileAsync and compileStringAsync.
An array of arguments passed by the function's caller. If the function takes arbitrary arguments, the last element will be a SassArgumentList.
The function's result. This may be in the form of a Promise, but
if it is the function may only be passed to compileAsync and compileStringAsync, not compile or compileString.
any - This function may throw an error, which the Sass compiler will
treat as the function call failing. If the exception object has a message
property, it will be used as the wrapped exception's message; otherwise, the
exception object's toString() will be used. This means it's safe for custom
functions to throw plain strings.
A callback that implements a custom Sass function. This can be passed to Options.functions.