Tidy up the Makefile a bit

This removes the duplication of the screen cache cleaning, and also turns it
into a target so you can do it yourself from the command line if needed. It
also marks all the phony targets as such (that is, targets that aren't
actually items in the filesystem).

Makefile

@@ -1,22 +1,43 @@
+.PHONY: test
 test:
 	pytest --cov-report term-missing --cov=textual tests/ -vv
+
+.PHONY: unit-test
 unit-test:
 	pytest --cov-report term-missing --cov=textual tests/ -vv -m "not integration_test"
+
+.PHONY: test-snapshot-update
 test-snapshot-update:
 	pytest --cov-report term-missing --cov=textual tests/ -vv --snapshot-update
+
+.PHONY: typecheck
 typecheck:
 	mypy src/textual
+
+.PHONY: format
 format:
 	black src
+
+.PHONY: format-check
 format-check:
 	black --check src
-docs-serve:
+
+.PHONY: clean-screenshot-cache
+clean-screenshot-cache:
 	rm -rf .screenshot_cache
+
+.PHONY: docs-serve
+docs-serve: clean-screenshot-cache
 	mkdocs serve --config-file mkdocs-online.yml
+
+.PHONY: docs-build
 docs-build:
 	mkdocs build --config-file mkdocs-online.yml
+
+.PHONY: docs-build-offline
 docs-build-offline:
 	mkdocs build --config-file mkdocs-offline.yml
-docs-deploy:
-	rm -rf .screenshot_cache
+
+.PHONY: docs-deploy
+docs-deploy: clean-screenshot-cache
 	mkdocs gh-deploy