build(deps): upgrade deps

date-fns: 2.14.0
vue-router: 3.3.4
vuex: 3.4.0
@tailwindcss/ui: 0.3.0
@vue/cli-plugin-babel: 4.4.6
@vue/cli-plugin-eslint: 4.4.6
@vue/cli-service: 4.4.6
sass: 1.26.9
tailwindcss: 1.4.6

remove @fullhuman/postcss-purgecss, use tailwindcss built-in purge option instead

Author: SilentDepth