Add an article on migrating package.json to "type: module"

[platosha]

The new default (Since 2.1) in Hilla projects is to have "type": "module" and "moduleResolution": "bundler" in package.json and tsconfig.json respectively.

The old setup without those changes should keep working, but let us document the recommended migration step to adopt the new defaults.

Doing so probably involves changing imports to use .js. We can suggest using ESLint import/extensions rule to help checking
and migrating the code.