feat: add rule widget (#3209)

* feat: add rule widget

* add star to init

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>

* remove unnecessary validations

* update rule styles

* add tests for invalid rules

* add minimum heights and widths

* tidy up examples

* remove old example

* move examples styling to tcss

* modify examples to fit docs screenshots

* add docs first draft

* add snapshot tests

* add rule to widget gallery

* make non-widget rule classes available

* tentatively update changelog

---------

Co-authored-by: Rodrigo Girão Serrão <5621605+rodrigogiraoserrao@users.noreply.github.com>
Co-authored-by: Will McGugan <willmcgugan@gmail.com>

docs/examples/widgets/horizontal_rules.py

@@ -0,0 +1,27 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Rule, Label
+from textual.containers import Vertical
+
+
+class HorizontalRulesApp(App):
+    CSS_PATH = "horizontal_rules.tcss"
+
+    def compose(self) -> ComposeResult:
+        with Vertical():
+            yield Label("solid (default)")
+            yield Rule()
+            yield Label("heavy")
+            yield Rule(line_style="heavy")
+            yield Label("thick")
+            yield Rule(line_style="thick")
+            yield Label("dashed")
+            yield Rule(line_style="dashed")
+            yield Label("double")
+            yield Rule(line_style="double")
+            yield Label("ascii")
+            yield Rule(line_style="ascii")
+
+
+if __name__ == "__main__":
+    app = HorizontalRulesApp()
+    app.run()