fix(unstable): don't error on non-existing attrs or type attr

[marvinhagemeister]

When running selectors for JS linting plugins we would error when encountering an unknown attribute name:

// selector
Foo[non-existant]

// error
Error: Missing string id: <number>

This was caused by using 0 as the invalid marker, but also overloading 0 with an actual node type. So the fix is to reserve 0 as the invalid marker and move the property type to the next index.