Rework Shulker Box Folder Settings

[OpenBagTwo]

Summary

Resolves #105

List of Changes

• Centralizes all EnderChest configuration defaults into one single _DEFAULTS lookup
• Anything listed in _DEFAULTS can now be overwritten in enderchest.cfg, including the list of default folders inside a shulker box and the "standard" and "global" lists of link-folders
  • To this end, "Standard" is now "Global" and "nothing" is now the "Standard"
    • The docs have been updated to reflect this
  • Additionally, "Standard" is now default in the shulker box crafting prompt
• Resolved some TODOs about unnecessarily repeated EnderChest loading
• Added the litematica folders to the "global" set of link folders, resolving Add litematica to the "standard" link folders #105

Tech Debt and Other Concerns

Since these folder settings aren't written to the config if they're not already in the config, it might be worth creating a definitive configuration guide. Then again, #80 is another way to solve this issue.

Validation Performed

• Added an explicit test to make sure config/litematica is a link-folder in a "global" shulker box
• Updated my enderchest.cfg and confirmed that everything is looking good and roundtripping successfully
• Went through the enderchest craft shulker_box prompt a few times until I was satisfied with the prompt message and the general user experience of specifying global vs. standard vs. none vs. manual

PR Type

• This PR introduces a breaking change (will require a bump in the minor version)
• The changes in this PR are high urgency and necessitate a hotfix or patch
  release (will require rebasing off of release)
• This is a release (staging) PR (maintainer use only)

Checklist:

• I have read the contributor's guide
• I have run mkdocs serve locally and ensured that all API docs and
  changes I have made to the static pages are rendering correctly, with all links
  working
• All tech debt concerns have been resolved, documented as issues, or otherwise
  accepted
• I agree to license my contribution to this project under
  the GNU Public License v3

[OpenBagTwo]

Between #80 and 107e872 I'm "accepting" the tech debt (not opening an issue for it)