readline: refactor readline module

Refactor parts of the readline module

• Move to more efficient module.exports = {} pattern
• Restructure imports and other minor code cleanups
• Move CSI (escape codes) into internal/readline for easier maintenance
• Improve test coverage

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

readline