Upgrade to Nuxt 3

[lewisgoddard]

Replaces

• Bump nuxt from 2.15.8 to 3.4.3 #99
• Bump eslint from 8.57.0 to 10.0.0 #329
• Bump stylelint from 15.11.0 to 17.3.0 #333
• Bump sass-loader from 10.4.1 to 17.0.0 #382 (removed)
• and about 30 sub-dep alerts.

Config Changes

• .eslintrc replaced with eslint.config.mjs
• nuxt.config.js replaced with nuxt.config.ts
• Added typecheck to CI with tsconfig.json

Code Changes

• .scss - Fix @import is deprecated.
• Move from custom module to @nuxtjs/plausible
• components/ + layouts/ - Use new Nuxt/Vue 3 format and TypeScript.