* feat: Add a symbol option to `battery.display`
* feat: Add a symbol option to `battery.display`
* use `impl defaulat` instead of `RootModuleConfig`
* edit the code according to clippy's linting
* change variable type to `Option<'a str>`
* update the documentation on the battery module
* updated documentation and source code according to review comment
* remove the unnecessary method and write the default value of BatteryDisplayConig to the document
* add 'charging_symbol' option to battery.display
Implements the Default trait for SegmentConfig to clean up construction of empty segments.
Also adds a segment::new() function to ease construction of simple segments.