perf(scrollbar): const creation

[EdJoPaTo]

While doing other things in the Scrollbar widget I used a bunch of lints to improve things. A bunch of const fn allow for more performance and Default now uses the const new implementations.

For simplicity reasons I created Scrollbar::new_with_symbols for a cleaner new without calling orientation() on new.

[codecov]

Codecov Report

Attention: Patch coverage is 62.29508% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 91.7%. Comparing base (d0067c8) to head (f4cda78).

Files	Patch %	Lines
src/widgets/scrollbar.rs	62.2%	23 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #963     +/-   ##
=======================================
- Coverage   91.9%   91.7%   -0.3%     
=======================================
  Files         61      61             
  Lines      15729   15742     +13     
=======================================
- Hits       14462   14440     -22     
- Misses      1267    1302     +35     

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[Valentin271]

This looks good, thank you!