Rewrite monorepo install script

[tbradsha]

While working through the setup docs, I tried to use the install script for the first time, and ran into dozens of hiccups on Mac on Linux. It also previously ran a lot of extra commands when unneeded, and didn't consistently set up the environment across shells. I went ahead and rewrote the install script; it's a bit verbose, but should be a bit more usable.

I don't account for all edge cases, but it should be robust enough, and should readily exit if something looks broken.

Given we run this in as an executable (i.e. in a subshell), restarting the terminal is required if environmental vars are changed. This was always the case.

Does this pull request change what data or activity we track or use?

Nope.

Testing instructions:

If you already have everything set up, you should be able to run the script and it should run through without any needed changes.

However, feel free to try on a stock macOS or Debian-based distro and ensure things run smoothly.