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/api/worker.md

@@ -0,0 +1 @@
+::: textual.worker

docs/api/worker_manager.md

@@ -0,0 +1 @@
+::: textual._worker_manager