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 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 the construct-style-sheets polyfill:

construct-style-sheets-polyfill 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 https://github.com/vaadin/flow/issues/12574

[Lodin]

Unfortunately, that doesn't seem possible since it breaks all the configuration for Rollup and Karma. While Rollup can be fixed by migrating to ES2015+, Karma doesn't allow it since it doesn't support ESM.

The only choice I can see is to provide additional .mjs file that is described in exports field of package.json. That would fix the Vite issue (and issue for all the modern development stack) along with preserving the current configuration.

I'm going to take care of it.