#1028 #1029 Possible first implementation for additional default eleventy config files

[Snapstromegon]

Currently only .eleventy.js is supported as the default config file.

This PR implements a possible solution for multiple config file. It intentionally just implements support for multiple default config files.

The implementation might be a little flaky around unittests, but it passes all of them consistently.

A future PR should probably look into allowing usage of multiple config files at once. This only uses the first existing one.

fixes #1028 fixes #1029