Migrate from LESS to SCSS for preprocessing

[rsek]

resolves #567

WIP -- most of the migration is done, just have to hunt down the remaining broken bits and rewire them for SCSS. the bulk of the new code is additions to package-lock.json; the vast majority of the other file changes are renames, syntax adjustments for the new preprocessor, and some reordering by the new stylelint configuration.

no joy on running SASS through PostCSS -- i couldn't get it to work with custom function plugins. i might revisit that later. for the moment, SCSS compilation uses a more standard Vite config.

TODO

• run conversion script for bulk migration of SCSS files
• pick over all files for stuff that didn't migrate, or didn't migrate properly.
• develop a sensible stylelint config and apply it
• add some basic path aliases and auto-imports so that SFCs can import mixins with minimum grief
• rewrite palettize in SCSS + JS, to take advantage of chroma-js's fancy color spaces for smarter color mixing
  • had to write a custom plugin for this. which isn't just scope creep -- SCSS doesn't have a true replacement for some of the more esoteric color functions i use in pallettize.
• hunt down remaining migration errors (mostly box shadows and gradients)
  • SF legacy tracks now show their hover effect more reliably, and will have theme-appropriate colours generated for them at build time (via a mixin)
  • fix progress animation
  • fix roll dialog buttons (and other places where there's non-vue buttons in ironsworn applications)
• use custom plugin instead of manual key addition to handle icon mask generation deferring to a later PR
• Update CHANGELOG.md