refactor!: project maintanace

[antfu]

Changes

• Switched to pnpm
• Using @akryum/sheep and @akryum/release-tag
• Move demos out as /demos
• Upgrade dependencies (close #936)
• Use vue-tsc to generate dts instead of vue-typegen
• Migrated to VitePress
• Move docs to the root

Todo

• [ ] Merge vue2 support to the same branch

[r4fx]

Nice work! 👏🏻 When can we expect new release?

[billbravo99]

why not update to composition api ? @antfu