Worker API (#2182)

* worker class

* worker API tests

* tidy

* Decorator and more tests

* type fix

* error order

* more tests

* remove active message

* move worker manager to app

* cancel nodes

* typing fix

* revert change

* typing fixes and cleanup

* revert typing

* test fix

* cancel group

* Added test for worker

* comment

* workers docs

* Added exit_on_error

* changelog

* svg

* refactor test

* remove debug tweaks

* docstrings

* worker test

* fix typing in run

* fix 3.7 tests

* blog post

* fix deadlock test

* words

* words

* words

* workers docs

* blog post

* Apply suggestions from code review

Co-authored-by: Dave Pearson <davep@davep.org>

* docstring

* fix and docstring

* Apply suggestions from code review

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

* Update src/textual/widgets/_markdown.py

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

* Apply suggestions from code review

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

* Update src/textual/worker.py

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

* Fix black

* docstring

* merge

* changelog

---------

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

docs/examples/guide/workers/weather03.py

@@ -0,0 +1,42 @@
+import httpx
+from rich.text import Text
+
+from textual import work
+from textual.app import App, ComposeResult
+from textual.containers import VerticalScroll
+from textual.widgets import Input, Static
+
+
+class WeatherApp(App):
+    """App to display the current weather."""
+
+    CSS_PATH = "weather.css"
+
+    def compose(self) -> ComposeResult:
+        yield Input(placeholder="Enter a City")
+        with VerticalScroll(id="weather-container"):
+            yield Static(id="weather")
+
+    async def on_input_changed(self, message: Input.Changed) -> None:
+        """Called when the input changes"""
+        self.update_weather(message.value)
+
+    @work(exclusive=True)
+    async def update_weather(self, city: str) -> None:
+        """Update the weather for the given city."""
+        weather_widget = self.query_one("#weather", Static)
+        if city:
+            # Query the network API
+            url = f"https://wttr.in/{city}"
+            async with httpx.AsyncClient() as client:
+                response = await client.get(url)
+                weather = Text.from_ansi(response.text)
+                weather_widget.update(weather)
+        else:
+            # No city, so just blank out the weather
+            weather_widget.update("")
+
+
+if __name__ == "__main__":
+    app = WeatherApp()
+    app.run()