feat: recursive config search

[pd93]

Fixes #2014

This PR changes how the experiments package initialises experiments. It allows them to be enabled by calling experiments.Parse() instead of relying on an init() function. This solves the issues raised in #2014, but also makes it easier to support loading experiments from other sources (such as config files).

Fixes #2159

The code to load a .taskrc.yml config has been moved out of the experiments package to make it more generic (it may be used for other things in the future). A new taskrc package has been created for this purpose that aligns with the equivalent taskfile package for loading Taskfiles. They have a very similar structure, although taskrc doesn't need to be anything like as complicated as the concepts of includes, remote files, node types and caching are all irrelevant at this time.

Some of the utilities used by both the taskfile and taskrc packages have been pulled out into a new fsext package. The advantage of this is that taskrc now gets access to powerful functions that let us search dynamically for files recursively up the file tree.

I have also (as usual) tried to tidy up a few bits of code as I went.