Changes the default for preloading pins

[codergeek121]

Before this PR, you'd have to opt-in to preloading pins. Now you have to opt out for some pins.

If this PR is too big of a breaking change, I could also introduce a setting, which can be toggled in a major release:

# engine.rb
config.importmap.preload_by_default = true

I'd be happy to adapt 👍

[dhh]

Awesome. Yes I don’t want to change a default and surprise people. Let’s add the toggle and we can default that in next Rails. Thanks!

[codergeek121]

I adapted the PR as discussed to just include a config for the preloading default without changing the current behavior.

A little bit OT:
I struggled a bit with the testsuite, because one service had connectivity issues. If webmock is a thing that this gem would adapt for the tests, I could look into integrating it and try to make the suite more reliable.

[dhh]

On second thought, there's a bunch of defaults I want to change for version 2.0, so I've just rolled this in with that in its pure form: #218