refactor!: ship ESM-only builds (#2544)

Author: joshuaellis

.changeset/esm-only.md

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+---
+'@react-spring/web': major
+---
+
+refactor!: ship ESM-only builds
+
+Every package now publishes a single modern ESM bundle (`dist/<name>.modern.mjs`) and drops its CommonJS output, the Webpack 4 `legacy-esm` fallback, and the orphaned pre-compiled dev/prod variants.
+
+ESM consumers (Vite, webpack 5, Next, esbuild, Bun, Deno, native Node ESM) are unaffected. CommonJS consumers must migrate to ESM or run on Node ≥22.12, where `require()` of an ESM package is supported. Webpack 4 is no longer supported.