feat: refactor book import logic to be more consistent and user-friendly

[bandantonio]

Description

This update includes:

• Refactor of single book import logic that does not overwrite existing highlights
• Dialog box to confirm overwrite of existing book highlight(s) for cases when backups are disabled
• Documentation describing the updated behavior
• Updated tests

Fixes #34

Checklist

• I accept the Code of Conduct
• I have performed a self-review of my code for consistency and potential leftover debug logs, commented out code, etc.
• I have added tests that prove that my feature works or that the fix is effective
• I have checked that new and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (if necessary)
• I confirm that this PR DOESN'T change the plugin's version either in the manifest.json or package.json files
• I have checked that my commit messages are descriptive and follow the Conventional Commits standard
• I have enabled the checkbox to allow maintainer edits so the branch can be updated for a merge.