Add changeset for tsdown migration

Added comprehensive changeset documenting:
- Breaking changes (dts option removal, config restructuring)
- New features (reactCompiler, tsdown.tsgo)
- Performance improvements (~2x faster builds)
- Dependency changes

Co-authored-by: stipsan <[email protected]>

Author: Copilot

.changeset/tsdown-migration.md

@@ -0,0 +1,31 @@
+---
+"@sanity/pkg-utils": major
+---
+
+Migrate from rollup/rolldown to tsdown for bundling
+
+## Breaking Changes
+
+- Removes `dts: 'api-extractor'` and `dts: 'rolldown'` config options - tsdown handles DTS automatically
+- Removes `rollup` configuration object from config types  
+- Moves `babel.reactCompiler` to top-level `reactCompiler` option
+- Moves `tsgo` option to `tsdown.tsgo`
+
+## Features
+
+- Added `reactCompiler: boolean | {target: '18' | '19'}` config option for React Compiler support
+- Added `tsdown.tsgo` option for native TypeScript preview via `@typescript/native-preview`
+- Implemented React Compiler using `@rollup/plugin-babel` as recommended by tsdown
+- Replaced esbuild-based export validation with publint
+- Consolidated JS and DTS generation under tsdown
+- Retained api-extractor for TSDoc validation
+
+## Performance
+
+- Faster builds (~421ms vs previous ~800ms for pkg-utils)
+- Simplified codebase (removed ~1200 lines of rollup/rolldown code)
+
+## Dependencies
+
+- **Added**: `[email protected]`, `publint@^0.3.15`, `@babel/preset-react@^7.28.5`
+- **Removed**: `rollup`, `rolldown`, `rolldown-plugin-dts`, `esbuild`, `browserslist-to-esbuild`, and rollup plugins (except `@rollup/plugin-babel` for React Compiler)