Add support for Sanity v5 and React 19

[torleifhalseth]

Summary

@frontify/sanity-plugin-frontify@1.0.0 (the published version) declares peer dependencies on react@^18 and sanity@^3, but the current Sanity ecosystem has moved to Sanity v5 and React 19.

Current state on main

The main branch has already addressed some of this:

• ✅ react peer dep widened to ^18 || ^19.1.0
• ✅ @sanity/ui dependency updated to ^3.0.0
• ❌ sanity peer dep still pinned to ^3

These fixes haven't been published to npm yet (latest is still 1.0.0).

Problem

When installing the published plugin in a Sanity v5 / React 19 project, pnpm reports unmet peer dependencies:

apps/studio
└─┬ @frontify/sanity-plugin-frontify 1.0.0
  ├── ✕ unmet peer react@^18: found 19.2.4
  └── ✕ unmet peer sanity@^3: found 5.19.0

Request

1. Widen the sanity peer dep to ^3 || ^4 || ^5 (see fix: widen sanity peer dependency to support v4 and v5 #30)
2. Publish a new release to npm with the React 19, Sanity v5, and @sanity/ui@3 support that's already on main