Config loading and hierarchical merging

[amyreese]

Summary

First piece of config loading based on the proposal in https://github.com/amyreese/Fixit/tree/config-prototype

• Walks the tree up from a target path to either a config with root = true or the top of the drive/filesystem.
• Loads all discovered files using tomllib/tomli.
• Basic priority-based overrides of values from outer configs.

Future work will add support for more intelligent merging of values rather than strict overrides, as well as support for multiple path-based overrides in a single config.

Test Plan

Tests!