Be consistent with naming of dmenu-extended

[MarkHedleyJones]

I have come across the following ways of naming dmenu-extended

• dmenuExtended (part of the name of the preferences file)
• dmenu-extended (the cononical name of the project)
• dmenu_extended (filenames used to execute dmenu-extended from the terminal, and the project name in pyproject.toml)

These should be made more consistent but in a way that preserves users' existing configurations.