* 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.

- add jest as extension devDep, go back to using makefile foreach syntax

- add correct exit code

- add yarn check call to install-deps

- add yarn invokation to find deps
Signed-off-by: Sebastian Malton <sebastian@malton.name>