feat(viewport): add content style for controlling colors within viewport

[Rydez]

There's a weird issue when attempting to change the colors within the viewport when the using reflow/wrap.

Here's a code example:

var c1 = lipgloss.Color("111")
var c2 = lipgloss.Color("123")
var c3 = lipgloss.Color("239")

func main() {
	const name = "somebody"
	const msg = "a nice message about some things."

	const limit = 20

	nameStyle := lipgloss.NewStyle().Foreground(c1).Background(c3)
	msgStyle := lipgloss.NewStyle().Foreground(c2).Background(c3)
	wrapped := wrap.String(nameStyle.Render(name+": ")+msgStyle.Render(msg), limit)

	vp := viewport.New(20, 5)

	vpStyle := lipgloss.NewStyle().Foreground(c2).Background(c3)
	vp.Style = vpStyle

	vp.SetContent(wrapped)

	fmt.Println(vp.View())
}

which results in:

However, with this PR and changing the code above like so:

-	vp.Style = vpStyle
+	vp.ContentStyle = vpStyl

the result is:

I struggled to find a way to get this result without changing viewport.go.