qmllint: add fixed warning ids

Currently our warnings are rather free-form. We should consider giving them ids (either numbers or names):

The advantage would be that they

1. allow you to
   have a table with more details like https://doc.qt.io/qtcreator/creator-checking-code-syntax.html#javascript-and-qml-error-codes,
2. can be parsed in an IDE if you add them as a special JSON field
   and lastly,
3. would even allow some dedicated pages like doc.qt.io/qmllint/issues/<ID> with a description of the issue and a recommended way how to fix them (basically, a better version of 1)

For prior art, see https://doc.rust-lang.org/error-index.html for how Rust does it