refactor: dont manually impl Default for defaults

[EdJoPaTo]

Replace impl Default by #[derive(Default)] when its implementation equals.

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Funny finding, the default Canvas doesn't use the default Marker:

https://github.com/ratatui-org/ratatui/blob/4f7791079edd16b54dc8cdfc95bb72b282a09576/src/widgets/canvas.rs#L621

https://github.com/ratatui-org/ratatui/blob/4f7791079edd16b54dc8cdfc95bb72b282a09576/src/symbols.rs#L489-L492

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.2%. Comparing base (4f77910) to head (5bb7a75).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1142   +/-   ##
=====================================
  Coverage   94.2%   94.2%           
=====================================
  Files         60      60           
  Lines      14623   14597   -26     
=====================================
- Hits       13780   13760   -20     
+ Misses       843     837    -6     

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