build: add dual package support for ESM and CommonJS

[tommyo]

• Update package.json with dual exports configuration (ESM/CJS)
• Simplify rollup config to output both module formats
• Remove browser-specific build configuration
• Update TypeScript config with ES2018 target and ESNext module
• Standardize code formatting (spacing in imports/types)
• Bump version to 3.1.2

This change enables the package to be consumed by both ESM and CommonJS projects through conditional exports, improving compatibility across different module systems.

[grimmdude]

Thanks for adding dual ESM/CJS package support — the package.json exports field looks correctly structured. A few issues I'd like addressed before merging:

1. Breaking type change to NoteOnEvent.pitch

NoteOnEvent is a public export, and the pitch type has been narrowed from string|string[]|number|number[] to string|number. External consumers who construct NoteOnEvent directly with array pitches will get a type error after this change. If this narrowing is intentional it should be split into a separate PR with an appropriate version bump and migration note — not bundled into a patch release.

2. Browser/IIFE build silently removed

The entire browser build (browser/midiwriter.js, build/index.browser.js) and @rollup/plugin-replace have been removed. This is a breaking change for anyone consuming the library via <script> tags. The PR description mentions it but this warrants more prominent treatment and a major version bump.

3. Version bump should reflect breaking changes

Given the type narrowing (#1) and browser build removal (#2), a patch bump (3.1.1 → 3.1.2) understates the scope of these changes. This should be at least a minor bump, or major if the browser build removal is considered part of the public API.

4. ESM output missing exports field in Rollup config

The CJS output specifies exports: 'default' but the ESM output has no exports field. Since main.ts uses a default export, this should be explicitly set for consistency and to avoid ambiguity. Please verify the test suite passes against both output formats.