fix: correct dual-package hazard in package exports

[Joeri van Oostveen (jvanoostveen)]

package.json declared "type": "module" but "main" still pointed to the
CommonJS build (dist/maquette.cjs.js). Because the file extension was
.js, Node/Vite parsed it as ESM despite its CommonJS content, causing
"ReferenceError: exports is not defined" in consumers.

Rename the CJS build output to dist/maquette.cjs (unambiguous
CommonJS regardless of "type": "module") and add an explicit exports
map so import/require resolve to the correct build.

[Copilot]

Pull request overview

Fixes a dual-package resolution hazard where the package is marked "type": "module" but previously pointed "main" at a CommonJS build with a .js extension, causing some consumers to parse the CJS file as ESM and fail at runtime.

Changes:

• Rename the CommonJS entrypoint from dist/maquette.cjs.js to dist/maquette.cjs via the "main" field.
• Add an explicit "exports" map so import and require resolve to the correct builds (and expose ./package.json).

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[Clément Billoré (darksabrefr)]

To create a fully functional ESM library in Node, you also need to explicitly add the .js extensions in imports in all source files. Extensionless imports are supported by bundlers but not by Node in ESM mode.