Adds support for loading oxvg configuration from common dot-config sources, e.g. ~/.config/oxvg

Closes #177

Description

Replaces config with etcetera for lighter-weight and more extensive config resolution. Previously the config would only be picked up from cwd, the config can now also be loaded from common dot-file sources.

Motivation and Context

This allows users to easily provide a common configuration when using oxvg in various places.

How Has This Been Tested?

Running oxvg optimise --config to preview each situation

• Created a config at ./oxvgrc.json
• Created a config at ~/.config/oxvg/config.json.
• Tested with a malformed config.
• Tested with no configs (i.e. default config)

Types of changes

Features

• Config can be loaded from ~/.config/oxvg/config.json on XDG/Unix, ~/Library/Preferences/oxvg/config.json on Apple, and ~\AppData\Roaming\oxvg\config.json on Windows.

Breaking changes

• json5 no longer supported
• Panics when resolved config is malformed

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.