refactor: switch from tsc to tsup for build

[jackmisner]

• Replace separate ESM/CJS/types builds with tsup which handles all three in a single step and properly bundles code without relative import issues.

• This fixes ESM module resolution errors when importing from projects that use Node.js ESM (missing .js extensions on relative imports).

Summary by CodeRabbit

• Chores
  • Refactored build system for improved efficiency and maintainability.
  • Standardised package distribution paths across all export formats.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Walkthrough

The build system is migrated from separate build scripts for ESM, CommonJS, and type declarations to a unified tsup-based builder. Package exports are restructured to use a flatter directory layout with explicit file extensions (.cjs, .js, .d.ts, .d.cts).

Changes

Cohort / File(s)	Summary
Build Configuration tsup.config.ts	New tsup configuration file defining entry points (src/index.ts, src/react/index.ts), dual format output (ESM and CommonJS), type declaration generation, sourcemap output, and tree-shaking with external React dependencies.
Package Configuration package.json	Updated main, module, and types fields to point to new flat dist paths. Exports map restructured: root and ./react entries now map directly to dist files using .cjs and .js extensions rather than nested cjs/esm/types directories. Build scripts consolidated into single tsup invocation; build:esm, build:cjs, and build:types removed. Added tsup as devDependency.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and accurately describes the main change: replacing the build tool from tsc to tsup, which is the primary objective of this pull request.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}