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.

textarea/textarea.go

@@ -283,12 +283,12 @@ type Model struct {
 }
 
 // New creates a new model with default settings.
-func New() Model {
+func New(isDark bool) Model {
 	vp := viewport.New(0, 0)
 	vp.KeyMap = viewport.KeyMap{}
 	cur := cursor.New()
 
-	styles := DefaultStyles(true)
+	styles := DefaultStyles(isDark)
 
 	m := Model{
 		CharLimit:            defaultCharLimit,

textarea/textarea_test.go

@@ -1703,7 +1703,7 @@ func TestView(t *testing.T) {
 }
 
 func newTextArea() Model {
-	textarea := New()
+	textarea := New(true)
 
 	textarea.Prompt = "> "
 	textarea.Placeholder = "Hello, World!"