fix: change module type to "module"

[ocavue]

Closes #71

Update package.json to matches ProseMirror's official packages (e.g. prosemirror-history/package.json):

• Add "type": "module"
  • Before: index.js is Common JS, index.mjs is ESM
  • After: index.js is ESM, index.cjs is Common JS
• Add "exports": { ... }
• Add "sideEffects": false. Good for tree shaking.
• Move prosemirror-* from peerDependencies to dependencies.

Update the build tools from Rollup to tsdown. The configuration is simpler now.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	11.49%	621 / 5402
🔵	Statements	11.49%	621 / 5402
🔵	Functions	47.72%	21 / 44
🔵	Branches	62.75%	91 / 145

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
tsdown.config.mjs	0%	0%	0%	0%	1-11

Generated in workflow #172 for commit 07b6fc0 by the Vitest Coverage Report Action