chore: set type to module in package.json

[vursen]

The type property of package.json may be used by frontend tooling (e.g Webpack, Vite) in determining whether the package's entry point is a conventional CJS or ES module. If no property is specified, the entry point may be treated as a CJS module. As an illustration, here is a warning that Vite shows when you import vaadin-development-mode-detector:

vaadin-development-mode-detector doesn't appear to be written in CJS, but also doesn't appear to be a valid ES module (i.e. it doesn't have "type": "module" or an .mjs extension for the entry point). Please contact the package author to fix.

This PR explicitly sets the type property to module to avoid possible misinterpretation.

Related to vaadin/flow#12574