Compile-only CLI flag

[V0ldek]

Is your feature request related to a problem? Please describe.
When implementing new selectors or just diagnosing bugs with a particular query, we often have to look at the compilation itself and the resulting automaton. It would be nice to have a way to run the binary only to the point of compilation, and not involve any of the engines.

Describe the solution you'd like
A new variant to the --engine switch sounds ideal. Example run:

just r '$..a.b' -e compiler -v

DEBUG [rsonpath_lib::query::parser] Parsed tokens: $(Descendant("a"))(Child("b"))
INFO  [rsonpath] Preparing query: `$..['a']['b']`

DEBUG [rsonpath_lib::query::automaton] NFA: r1 --a-> d1 --b-> acc
DEBUG [rsonpath_lib::query::automaton::minimizer] New superstate created: {NfaStateId(0)} DFA(1)
DEBUG [rsonpath_lib::query::automaton::minimizer] Expanding superstate: {NfaStateId(0)}, last checkpoint is Some(NfaStateId(0))
DEBUG [rsonpath_lib::query::automaton::minimizer] Considering transition NFA(0) --"a"-> NFA(1)
DEBUG [rsonpath_lib::query::automaton::minimizer] Raw transitions: {"a": {NfaStateId(1)}}
DEBUG [rsonpath_lib::query::automaton::minimizer] New superstate created: {NfaStateId(0), NfaStateId(1)} DFA(2)
DEBUG [rsonpath_lib::query::automaton::minimizer] Normalized transitions: {"a": {NfaStateId(0), NfaStateId(1)}}
DEBUG [rsonpath_lib::query::automaton::minimizer] Translated transitions: [("a", State(2))]
DEBUG [rsonpath_lib::query::automaton::minimizer] Expanding superstate: {NfaStateId(0), NfaStateId(1)}, last checkpoint is Some(NfaStateId(0))
DEBUG [rsonpath_lib::query::automaton::minimizer] Considering transition NFA(0) --"a"-> NFA(1)
DEBUG [rsonpath_lib::query::automaton::minimizer] Considering transition NFA(1) --"b"-> NFA(2)
DEBUG [rsonpath_lib::query::automaton::minimizer] Raw transitions: {"a": {NfaStateId(1)}, "b": {NfaStateId(2)}}
DEBUG [rsonpath_lib::query::automaton::minimizer] New superstate created: {NfaStateId(0), NfaStateId(2)} DFA(3)
DEBUG [rsonpath_lib::query::automaton::minimizer] Normalized transitions: {"a": {NfaStateId(0), NfaStateId(1)}, "b": {NfaStateId(0), NfaStateId(2)}}
DEBUG [rsonpath_lib::query::automaton::minimizer] Translated transitions: [("a", State(2)), ("b", State(3))]
DEBUG [rsonpath_lib::query::automaton::minimizer] Expanding superstate: {NfaStateId(0), NfaStateId(2)}, last checkpoint is Some(NfaStateId(0))
DEBUG [rsonpath_lib::query::automaton::minimizer] Considering transition NFA(0) --"a"-> NFA(1)
DEBUG [rsonpath_lib::query::automaton::minimizer] Raw transitions: {"a": {NfaStateId(1)}}
DEBUG [rsonpath_lib::query::automaton::minimizer] Normalized transitions: {"a": {NfaStateId(0), NfaStateId(1)}}
DEBUG [rsonpath_lib::query::automaton::minimizer] Translated transitions: [("a", State(2))]
DEBUG [rsonpath_lib::stackless] DFA:
 digraph {
  0 -> 0 [label="*"]
  1 -> 2 [label="a"]
  1 -> 1 [label="*"]
  2 -> 2 [label="a"]
  2 -> 3 [label="b"]
  2 -> 1 [label="*"]
  3 -> 2 [label="a"]
  3 -> 1 [label="*"]
}
INFO  [rsonpath] Compilation finished.

Describe alternatives you've considered
A dedicated flag could be used, --compile (short -c). It would be mutually exclusive with -e. Since the run would be completely different we could have the binary just output the compiled automaton to stdout in .dot format, which would make inspecting it easier.

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[V0ldek]

We should go ahead with a -c/--compile flag. Clap supports mutually exclusive arguments, so we can say that either -e or -c is allowed, but not both.