feat: custom filenames for package files (#1894)

Renovate now comes with a variety of package managers supported, each with their own filename pattern(s). These patterns are now exposed for user configuration through the new `fileMatch` list/array configuration option, which has been added to each manager (npm, bazel, docker-compose, etc). `fileMatch` is defined as a mergeable list, meaning that users can add to the default pattern to extend the files being detected.

Closes #799