chore: Switch to github.com/go-sprout/sprout for internal tools

[twpayne]

Refs #2668.

This should be fully backwards-compatible.

Draft for now.

[marmitar]

I decided to test it and the biggest change I felt is with dig. In sprig you had dig ...PATH DEFAULT DICT and DEFAULT was required, but in sprout it's dig ...PATH DICT which returns nil if missing.

I think sprout's implementation is more natural, since I actually have forgotten about the default quite a few times before, writing something like dig ...PATH DICT | default DEFAULT. Anyway, just wanted to let you know about this change.

[twpayne]

Thank you very much for the testing! This PR needs more work, specifically:

• Currently this PR uses sprout's sprigin package, which should provide 100% compatibility with sprig. Therefore, there should be no change to the documentation in this PR.
• To migrate to sprout properly, users should be able to switch from sprig to sprout in their templates at their own pace. I'm not sure if this should be configured globally or on a per-file basis.

I'll update the PR to migrate to sprigin internally, which should have no user-visible changes.

[twpayne]

Thanks again for the testing. This PR now only touches internal tools and adds some tests for sprig's behavior. Actually migrating to sprout will require more work, as described in #2668 (comment).