Prompt user for action when duplicate files are detected

[hadynz]

This is an enhancement in response to this comment https://github.com/hadynz/obsidian-kindle-plugin/issues/237#issuecomment-1493212654.

Currently the plugin has a strong bias for never mutating any user data. If a highlights file exists, it will always create a duplicate and make the file name unique by appending a timestamp.

The feature request is to modify this behaviour and ask the user for action:

• Do you want to override existing file?
• Retain existing. Create a new file

The user should also be able to choose an option for the plugin to remember their choice. This option can be changed later on via the plugin's settings panel.