Make devtools deps extras, add note to internal ref docs, raise excep… (#579)

* Make devtools deps extras, add note to internal ref docs, raise exception when deps missing

* Ensure dev extras are installed in GitHub Actions

* Update lockfile

* Add extra docs note

* Add a TODO about adding link to devtools docs

* Add click to dev extras dependencies in pyproject

reference/_devtools.md

@@ -0,0 +1,14 @@
+# Devtools
+
+## Installation
+
+Using the Textual Devtools requires installation of the `dev` extra dependency.
+
+https://python-poetry.org/docs/pyproject/#extras
+
+## Running
+
+TODO: Note how we run the devtools themselves and how we run our Textual apps
+such that they can connect. Don't forget Windows instructions :)
+We might also add a link to the documentation from the exception that gets
+raised when the "dev" extra dependencies aren't installed.