feat: add a Styler interface for styling bubbles

This change introduces a Styler interface that allows users to define
custom styles for bubbles. The interface is implemented by the StylerFunc
type, which is a function type that takes a boolean argument and returns
a value of any type.

This allows a user to define a custom StylerFunc that returns a custom
style for a bubble based on the background color of the terminal.

help/help_test.go

@@ -10,7 +10,7 @@ import (
 )
 
 func TestFullHelp(t *testing.T) {
-	m := New()
+	m := New(true)
 	m.FullSeparator = " | "
 	k := key.WithKeys("x")
 	kb := [][]key.Binding{