Create outline for Opacity renderable

src/textual/renderables/opacity.py

@@ -0,0 +1,19 @@
+from rich.console import ConsoleOptions, Console, RenderResult, RenderableType
+
+
+class Opacity:
+    """Return a renderable with the foreground color blended into the background color.
+
+    Args:
+        renderable (RenderableType): The RenderableType to manipulate.
+        value (float): The opacity as a float. A value of 1.0 means text is fully visible.
+    """
+
+    def __init__(self, renderable: RenderableType, value: float = 1.0) -> None:
+        self.renderable = renderable
+        self.value = value
+
+    def __rich_console__(
+        self, console: Console, options: ConsoleOptions
+    ) -> RenderResult:
+        pass