(v2) Add focus status to textarea's setPromptFunc

[meowgorithm]

This gives focus status to the prompt. We may want to reexamine this and add more metadata.

// Before
func (m *Model) SetPromptFunc(promptWidth int, fn func(lineIndex int) string)

// After
func (m *Model) SetPromptFunc(promptWidth int, fn func(lineIndex int, focused bool) string)