Remove Makefile from all intree extensions

- Makefiles are not really common (rightly so) in JS projects and script
  entries in the package.json are good enough and what they're there
  for.

- Move to using yarn for installing dependencies and running scripts as
  that is what we mostly use for the rest of the project.

- Move from using make syntax to a dedicated script for building and
  testing extensions.

Signed-off-by: Sebastian Malton <sebastian@malton.name>

Makefile

@@ -56,10 +56,10 @@ else
 endif
 
 build-extensions:
-	$(foreach dir, $(wildcard $(EXTENSIONS_DIR)/*), $(MAKE) -C $(dir) build || exit $?; )
+	yarn intree-extensions $(EXTENSIONS_DIR) build
 
 test-extensions:
-	$(foreach dir, $(wildcard $(EXTENSIONS_DIR)/*), $(MAKE) -C $(dir) test || exit $?; )
+	yarn intree-extensions $(EXTENSIONS_DIR) test
 
 build-npm: build-extension-types
 	yarn npm:fix-package-version