Reorganize theme file structure

Problem/Motivation

Mercury's file structure grew haphazardly over the course of time. It should be cleaned up.

Proposed resolution

• /css/components/ should be moved under /src/ somewhere.
• /images/icons/ should be moved under /icons/ or maybe go away entirely.
• /lib/ contains one file and should go under /src/ somewhere.
• /public/ I think can be deleted?
• /scripts/ contains one file, which should probably go wherever the stuff under /vitePlugins/ ends up.
• /static/ may or may not be needed after we get the static Storybook build working on GitLab pages.
• /vitePlugins/ has some things that aren't Vite plugins. /vitePlugins/twingCustoms/ was supposed to contain only filters.js and functions.js, as those are custom Twig filters and functions to be used with Twing. The stuff in here, /lib/, and /scripts/ all needs to be organized sensibly. Also, any two-word folders that survive the reorganization should be in kebab-case, not camelCase.

Remaining tasks

• Come up with and implement better file structure
• Modify the Vite/Storybook build, the .info.yml, the .libraries.yml, and all relative import/include paths to work correctly with the new file structure