Overlay annotations to indicate if a signal is supposed to already exist or not.

Today you can use overlays to both add new signals as well refactor existing signals. This gives a potential problem, if you do a spelling error or if a signal has been renamed or removed in standard VSS catalog your overlay may not do what you expect.

I am thinking if vss-tools possibly should support an attribute `overlay: ['add'|'modify']` ( or similar like `new: True|False`) that could be used in overlays to describe if the overlay intend to add a new signal or change an existing signal. Then tooling could give a warning if the directive does not match reality.

```
Vehicle.Boddy.RefuelPosition:
datatype: string
type: attribute
allowed: ['ALT1','ALT2']
overlay: ['add'|'modify']
description: Location of the fuel cap or charge port.
```

No implementation files, tests, or entry points are named. Start by tracing how vss-tools parses and validates overlays, then determine how an add/modify annotation should be represented and checked against the standard VSS catalog; the work is done when mismatches produce a defined warning and covered behavior is tested.

Written by the indexing model from the issue text.