Filter selector – existential filters `[?@<path>]`

[V0ldek]

Is your feature request related to a problem? Please describe.
Support for filters with the following restrictions:

• no nesting
• no logical/boolean operations
• only existential tests

So strictly filters of the form [?@<path>].

Describe the solution you'd like
The solution requires a theoretical write-up and then implementation.

The current idea is to have a second DFA, the filter automaton, controlled by the main query automaton. The state of the filter automaton would determine if a given path match is to be emitted, ingored, or stored to potentially be emitted later.

Additional context
RFC draft.

[github-actions]

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