feat: provide help text on invalid command if available

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.24324% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 94.95%. Comparing base (22e1a62) to head (caffe84).

Files	Patch %	Lines
rsql_core/src/shell/repl.rs	92.85%	10 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #75 +/- ## ========================================== + Coverage 94.39% 94.95% +0.56% ========================================== Files 54 54 Lines 5511 5612 +101 ========================================== + Hits 5202 5329 +127 + Misses 309 283 -26 ```

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Bencher

Report	Sun, March 24, 2024 at 21:42:03 UTC
Project	theseus-rs/rsql
Branch	invalid-command
Testbed	localhost

Benchmark	Latency	Latency Results nanoseconds (ns)
postgresql	➖ (view plot)	787990000.000
sqlite	➖ (view plot)	252480.000

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