Add make.ps1 cmdlet + activateenv command for Windows make scripts (#4087)

* Add missing command to `make.bat`'s help

* Add `activateenv` and `deactivateenv` commands to `make.bat`

* Add help to `Makefile`

* Add `make.ps1` cmdlet

Makefile

@@ -1,7 +1,28 @@
+.DEFAULT_GOAL := help
+
 PYTHON ?= python3.8
 
 ROOT_DIR:=$(shell dirname $(realpath $(firstword $(MAKEFILE_LIST))))
 
+define HELP_BODY
+Usage:
+  make <command>
+
+Commands:
+  reformat                   Reformat all .py files being tracked by git.
+  stylecheck                 Check which tracked .py files need reformatting.
+  stylediff                  Show the post-reformat diff of the tracked .py files
+                             without modifying them.
+  gettext                    Generate pot files.
+  upload_translations        Upload pot files to Crowdin.
+  download_translations      Download translations from Crowdin.
+  bumpdeps                   Run script bumping dependencies.
+  newenv                     Create or replace this project's virtual environment.
+  syncenv                    Sync this project's virtual environment to Red's latest
+                             dependencies.
+endef
+export HELP_BODY
+
 # Python Code Style
 reformat:
 	$(PYTHON) -m black $(ROOT_DIR)
@@ -29,3 +50,7 @@ newenv:
 	$(MAKE) syncenv
 syncenv:
 	.venv/bin/pip install -Ur ./tools/dev-requirements.txt
+
+# Help
+help:
+	@echo "$$HELP_BODY"