feat: support changing the cursor style

This commit adds support for changing the cursor style. The new
`SetCursorStyle` command can be used to change the cursor style to one of
the following:

- `CursorBlock`
- `CursorUnderline`
- `CursorBar`

cursor.go

@@ -8,3 +8,26 @@ type CursorPositionMsg struct {
 	// Column is the column number.
 	Column int
 }
+
+// CursorStyle is a style that represents the terminal cursor.
+type CursorStyle int
+
+// Cursor styles.
+const (
+	CursorBlock     CursorStyle = 1
+	CursorUnderline CursorStyle = 3
+	CursorBar       CursorStyle = 5
+)
+
+// setCursorStyle is an internal message that sets the cursor style.
+type setCursorStyle int
+
+// SetCursorStyle is a command that sets the terminal cursor style.
+func SetCursorStyle(style CursorStyle, blink bool) Cmd {
+	if !blink {
+		style++
+	}
+	return func() Msg {
+		return setCursorStyle(style)
+	}
+}

tea.go

@@ -426,6 +426,9 @@ func (p *Program) eventLoop(model Model, cmds chan Cmd) (Model, error) {
 					p.suspend()
 				}
 
+			case setCursorStyle:
+				p.execute(ansi.SetCursorStyle(int(msg)))
+
 			case modeReportMsg:
 				switch msg.Mode {
 				case graphemeClustering: