readline: add getPrompt to get the current prompt
Since there is a setPrompt() there should be a getPrompt(). There are use-cases where it is needed to know what the current prompt is. Adding a getPrompt() negates the need to store the set prompt externally or read the internal _prompt which would be bad practice.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines