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>

tests/test_work_decorator.py

@@ -0,0 +1,33 @@
+import asyncio
+
+from textual import work
+from textual.app import App
+from textual.worker import Worker, WorkerState
+
+
+async def test_work() -> None:
+    """Test basic usage of the @work decorator."""
+    states: list[WorkerState] = []
+
+    class WorkApp(App):
+        worker: Worker
+
+        @work
+        async def foo(self) -> str:
+            await asyncio.sleep(0.1)
+            return "foo"
+
+        def on_mount(self) -> None:
+            self.worker = self.foo()
+
+        def on_worker_state_changed(self, event: Worker.StateChanged) -> None:
+            states.append(event.state)
+
+    app = WorkApp()
+
+    async with app.run_test() as pilot:
+        await app.workers.wait_for_complete()
+        result = await app.worker.wait()
+        assert result == "foo"
+        await pilot.pause()
+    assert states == [WorkerState.PENDING, WorkerState.RUNNING, WorkerState.SUCCESS]