Add Styles.reset method revert internal attributes to None

tests/test_styles.py

@@ -0,0 +1,11 @@
+from rich.style import Style
+
+from textual.css.styles import Styles
+
+
+def test_styles_reset():
+    styles = Styles()
+    styles.text_style = "not bold"
+    assert styles.text_style == Style(bold=False)
+    styles.reset()
+    assert styles.text_style is Style.null()