QQmlLSUtils::resolveExpressionType() support for AttachedPropertyIdentifier

Introduce a new QQmlLSUtilsIdentifierType AttachedPropertyIdentifier to distinguish between attached properties (AttachedPropertyIdentifier) and attached types (AttachedTypeIdentifier). Currently, everything attached is a AttachedTypeIdentifier, which means that we can't distinguish "Keys" from "onPressed", for example on

Item { Keys.onPressed: ... }

Currently, one can do "find usages" on "onPressed" and get the right result, but "find usages" on "Keys" does not find usages of the "Key" attached property.