Support function varargs

Users should be able to create functions with variable arguments.

This already exists in the language, but is currently only available to the standard library (for example, `List` and `Map` accept varargs)

Depends on #1154

Start by reading this issue together with dependency #1154, then inspect how existing standard-library functions such as List and Map accept varargs. Done means users can define functions with variable arguments, matching the behavior already available to the standard library.

Written by the indexing model from the issue text.