Adjust RichLog.highlighter type to allow for custom highlighters.

Without an explicit type, tools like mypy assume RichLog.highlighter must be a ReprHighlighter instead of a Highlighter.

src/textual/widgets/_rich_log.py

@@ -5,7 +5,7 @@
 from typing import TYPE_CHECKING, Optional, cast
 
 from rich.console import RenderableType
-from rich.highlighter import ReprHighlighter
+from rich.highlighter import Highlighter, ReprHighlighter
 from rich.measure import measure_renderables
 from rich.pretty import Pretty
 from rich.protocol import is_renderable
@@ -86,7 +86,8 @@ def __init__(
         """Apply Rich console markup."""
         self.auto_scroll = auto_scroll
         """Automatically scroll to the end on write."""
-        self.highlighter = ReprHighlighter()
+        self.highlighter: Highlighter = ReprHighlighter()
+        """Rich Highlighter used to highlight content when highlight is True"""
 
         self._last_container_width: int = min_width
         """Record the last width we rendered content at."""