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.

bubbles.go

@@ -1 +1,15 @@
 package bubbles
+
+// Styler represents an interface for styling bubbles.
+type Styler[T any] interface {
+	Styles(isDark bool) T
+}
+
+// StylerFunc is a function type that implements the Styler interface.
+type StylerFunc[T any] func(isDark bool) T
+
+// Styles calls the function.
+// It implements the Styler interface.
+func (f StylerFunc[T]) Styles(isDark bool) T {
+	return f(isDark)
+}