readline: Allow history to be disabled

Checklist

• tests and code linting passes
• a test is included
• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

readline

Description of change

Fixes #6336 (closed)

1. The historySize to default to 30 only if undefined.
2. If historySize is set to 0, then disable caching the line.
3. Added unit tests.
4. Updated documentation.