Improve make script ergonomics & documentation

[pushfoo]

Changes

• Document how to enable running ./make.py instead of python make.py for Mac & Linux users
• Document how to enable tab completions for ./make.py
• Add a ./make.py whichshell sub-command to assist users with setting up tab completion

How to test

Both sets of instructions assume you are on Linux or a Mac

Testing short-form invocation & Shell Checking

1. Follow the instructions for enabling ./make.py
2. Run ./make.py whichshell to make sure it reports your default shell correctly

Testing Shell Completion

1. If you are running a supported shell, follow the included instructions
2. Type ./make.py h
3. Press tab
4. Observe the result; it should auto-complete to ./make.py html

[pushfoo]

1. I think the shebang line and chmod doc is definitely worth keeping
2. We can revert or split off the bash completion part to avoid conflict with Typechecking improvements #1738
3. Please provide feedback on which direction would be best