Add version option

src/textual/cli/cli.py

@@ -1,9 +1,11 @@
 import click
 
+import textual
 from textual.devtools.server import _run_devtools
 
 
 @click.group()
+@click.version_option(textual.__version__)
 def run():
     pass
 

tests/cli/test_cli.py

@@ -0,0 +1,10 @@
+from click.testing import CliRunner
+
+import textual
+from textual.cli.cli import run
+
+
+def test_cli_version():
+    runner = CliRunner()
+    result = runner.invoke(run, ["--version"])
+    assert textual.__version__ in result.output, "You need to update __version__"