Add ESM module support

[w8r]

Summary

This PR modernizes the package to support ES Modules (ESM) alongside the existing UMD format, enabling better tree-shaking and native module support in modern bundlers and Node.js.

Changes

• New ESM source file (src/leaflet-mapbox-gl.js) with native import/export syntax
• Rollup build pipeline producing both ESM and UMD bundles in dist/
• Package.json updates:
  • Added "type": "module" for native ESM
  • Added "module" field pointing to ESM build
  • Added "exports" field with conditional exports for ESM/CJS resolution
  • Added "types" field for TypeScript support
  • Added build scripts (npm run build, prepublishOnly)
• TypeScript definitions (dist/leaflet-mapbox-gl.d.ts)
• JSHint configuration (.jshintrc) updated for ES6+

Usage

ESM (modern bundlers/Node.js):

import { mapboxGL } from 'mapbox-gl-leaflet';
// or
import mapboxGL from 'mapbox-gl-leaflet';

CommonJS:

const { mapboxGL } = require('mapbox-gl-leaflet');

Browser (unchanged):

<script src="dist/leaflet-mapbox-gl.js"></script>
<script>
  L.mapboxGL({ style: '...' }).addTo(map);
</script>

Backwards Compatibility

• Original leaflet-mapbox-gl.js is kept in the package for users referencing it directly
• UMD bundle maintains the same L.mapboxGL / L.MapboxGL globals
• No breaking changes for existing users

Test plan

• npm run lint passes
• npm run build produces both ESM and UMD bundles
• Manual testing with ESM import in a bundler (Vite/webpack)
• Manual testing with script tag in browser

[Copilot]

Pull request overview

This PR adds an ES Module (ESM) build alongside the existing UMD distribution, aiming to improve compatibility with modern bundlers/Node ESM while preserving browser/global usage.

Changes:

• Added an ESM source entry (src/leaflet-mapbox-gl.js) with import/export and named/default exports.
• Introduced a Rollup build producing dist/leaflet-mapbox-gl.esm.js (ESM) and dist/leaflet-mapbox-gl.js (UMD).
• Updated package metadata/scripts and added TypeScript declarations.

Reviewed changes

Copilot reviewed 4 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/leaflet-mapbox-gl.js	New ESM entrypoint that exports MapboxGL / mapboxGL and also mutates L for compatibility.
rollup.config.js	Rollup config to emit ESM + UMD builds.
package.json	Adds conditional exports, sets type: module, and adds build/lint scripts.
package-lock.json	Updates lockfile to include Rollup dependencies and lockfile v3 format.
dist/leaflet-mapbox-gl.d.ts	Adds TypeScript typings and Leaflet module augmentation.
.jshintrc	Enables ES module linting and modern ECMAScript settings.
.gitignore	Ignores generated dist/*.js and sourcemaps while keeping .d.ts tracked.

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