Script command #2805

[snoyberg]

This adds a basic script command. Most of the work involved has to do
with preventing config files from being loaded. Originally we planned on
creating an alternative set of config types for with and without a local
config. That turned out to be prohibitively invasive. Instead, we now
just create a dummy config file instead ~/.stack/script/lts-x.y (or
/nightly-...).

While this addresses reproducibility, it doesn't help with the speed
concerns: script is now about 100ms faster than runghc on my system for
the case where --package is provided, but it's still over a second for
Hello World. The slowdown is inherent right now in checking if the
relevant packages are installed. It would be nice to figure out a way to
optimize the package check.

Also, this should include some integration tests. It should be a simple
matter of a test that includes a bogus stack.yaml and proving that stack
script ignores it.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

Manual testing so far. We should definitely add some test cases. I used the same example as from the GUIDE.md update:

#!/usr/bin/env stack
-- stack --resolver lts-6.25 script --package turtle
{-# LANGUAGE OverloadedStrings #-}
import Turtle
main = echo "Hello World!"

[mgsloan]

Seems good to me 👍

[decentral1se]

🍷