New CLI runner (#2338)

* New CLI runner

* runner functionality

* Add port

* use env for port

* changelog

* test fix

* flush

* remove constant

* comment

* tidy docs

* docstrings

* punctuation

* docstring

* fix test

* snapshot

* Update src/textual/cli/cli.py

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

* guard against bad imports

* guard againsts screenshot

* always print return

* docstrings

* docstrings

---------

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

tests/devtools/test_devtools_client.py

@@ -10,7 +10,7 @@ from rich.console import ConsoleDimensions
 from rich.panel import Panel
 
 from tests.utilities.render import wait_for_predicate
-from textual.constants import DEFAULT_DEVTOOLS_PORT
+from textual.constants import DEVTOOLS_PORT
 from textual.devtools.client import DevtoolsClient
 from textual.devtools.redirect_output import DevtoolsLog
 
@@ -22,7 +22,7 @@ TIMESTAMP = 1649166819
 
 def test_devtools_client_initialize_defaults():
     devtools = DevtoolsClient()
-    assert devtools.url == f"ws://127.0.0.1:{DEFAULT_DEVTOOLS_PORT}"
+    assert devtools.url == f"ws://127.0.0.1:{DEVTOOLS_PORT}"
 
 
 async def test_devtools_client_is_connected(devtools):