Add some initial component classes

This might be enough actually, still need to think on this some more. There
is another issue to think though here too. I'm leaning heavily on a base
"toggle" button class from which the checkbox and the radio button derive.
This makes a lot of sense -- they're so similar that it doesn't really make
sense to do it another way.

But... component classes lean on a common prefix; and I can't see an obvious
way of declaring component classes based off the owning class name; not at
the moment anyway. Perhaps I can make that work.

src/textual/widgets/_toggle.py

@@ -28,6 +28,19 @@ class ToggleButton(Static, can_focus=True):
     | enter,space | Toggle the value. |
     """
 
+    COMPONENT_CLASSES: ClassVar[set[str]] = {
+        "toggle--label",
+        "toggle--button-sides",
+        "toggle--button-inner",
+    }
+    """
+    | Class | Description |
+    | :- | :- |
+    | `toggle--label` | Targets the text label of the toggle button. |
+    | `toggle--button-sides` | Targets the side characters of the toggle button. |
+    | `toggle--button-inner` | Targets the inner character of the toggle button. |
+    """
+
     DEFAULT_CSS = """
     ToggleButton:hover {
         text-style: bold;
@@ -91,10 +104,12 @@ class ToggleButton(Static, can_focus=True):
     @property
     def _button(self) -> Text:
         """The button, reflecting the current value."""
+        side_style = self.get_component_rich_style("toggle--button-sides")
+        inner_style = self.get_component_rich_style("toggle--button-inner")
         return Text.assemble(
-            self.button_prefix,
-            (self.button_on if self.value else self.button_off),
-            self.button_suffix,
+            Text(self.button_prefix, style=side_style),
+            Text(self.button_on if self.value else self.button_off, style=inner_style),
+            Text(self.button_suffix, style=side_style),
         )
 
     def render(self) -> RenderResult:
@@ -104,8 +119,11 @@ class ToggleButton(Static, can_focus=True):
             The content to render for the widget.
         """
         button = self._button
-        label = self.label
-        spacer = " " if self.label else ""
+        label = Text(self.label, style=self.get_component_rich_style("toggle--label"))
+        spacer = Text(
+            " " if self.label else "",
+            style=self.get_component_rich_style("toggle--label"),
+        )
         return (
             Text.assemble(button, spacer, label)
             if self.button_first