Remove defaults from button theme & update border merge

[dickermoshe]

Closes #184

[nank1ro]

There is something wrong with your PR.
I ran the example, opened the Accordion page and it throws:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown building ShadDecorator(dirty, dependencies:
[ShadInheritedTheme]):
'package:shadcn_ui/src/theme/components/decorator.dart': Failed assertion: line 138 pos 12:
'BorderSide.canMerge(top, other.top)': is not true.

The relevant error-causing widget was:
  ShadDecorator
  ShadDecorator:file:///Users/ale/github/flutter-shadcn-ui/lib/src/components/switch.dart:174:22

When the exception was thrown, this was the stack:
#2      ShadBorder.mergeWith (package:shadcn_ui/src/theme/components/decorator.dart:138:12)
#3      ShadDecoration.mergeWith (package:shadcn_ui/src/theme/components/decorator.dart:303:35)

[nank1ro]

Are you sure the BorderSide.canMerge is needed?
Seems like something the Flutter team decided but I don't understand the reason.

/// Two sides can be merged if one or both are zero-width with
/// [BorderStyle.none], or if they both have the same color and style.

I want shadcn_ui to allow every kind of customizations, and not limit for the same style or color.

[dickermoshe]

Forgive me, I misunderstood what BorderSide.merge does. Turns out it's a pretty badly named function.
That function adds the 2 borders, instead of merging them.

Turns out you can't merge Borders, so defining one completely overwrites the previous one, even if you just want to change the color.

Gonna remove the changes and just keep the button stuff. Sounds Good?

[nank1ro]

Sounds good 👍

[dickermoshe]

done.