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>

.gitignore

@@ -15,4 +15,5 @@ src/extensions/*/*.d.ts
 types/extension-api.d.ts
 types/extension-renderer-api.d.ts
 extensions/*/dist
-docs/extensions/api
+extensions/*/package-lock.json
+docs/extensions/api