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_screens.py

@@ -1,4 +1,6 @@
+import asyncio
 import sys
+import threading
 
 import pytest
 
@@ -60,6 +62,8 @@ async def test_installed_screens():
 
 async def test_screens():
     app = App()
+    app._loop = asyncio.get_running_loop()
+    app._thread_id = threading.get_ident()
     # There should be nothing in the children since the app hasn't run yet
     assert not app._nodes
     assert not app.children