Add underscore to js evaluation to allow for use of get()

[macebake]

This will help with nested fields that could potentially be null

[LHeggy]

On Performance

after pulling up vm creation to an init function it is way way faster in every sense:

1.03s user 1.03s system 188% cpu 1.094 total

(tested on 1.5k entries)

On shared state

I reckon overwriting the subject each operation will protect us from any serious issues and further effort is not warranted because:

• Checked all 1.5k entries made and they match with entries in the database, so nothing is getting garbled by reusing the VM (at least in this simple use case)
• Someone went down the rabbit hole of making this safe a few years ago and it looks like they gave up. Although it seems like it was an academic concern even in their use case
• I can't imagine what a user would have to be doing with their jsonnet file to really start leaving nasty state in our otto VM. If that happens in the next few months I feel like we can tell them not to get clever with their jsonnet files and be a normal egg.

[macebake]

Something isn't quite right here - when I test locally on the backstage samples, only catalog types defined without a filter are synced. Filters are checked in output/collect.go

[LHeggy]

Something isn't quite right here - when I test locally on the backstage samples, only catalog types defined without a filter are synced. Filters are checked in output/collect.go

tweaked the jsonnet file, should be working now

[macebake]

did you mean to commit $badKey?

also, in these cases, we're sure that accessing nested field as normal still works? i assume so but wanted to check. We probably don't need all the examples of _.get() in the sample docs IMO (lmk what you think). Hopefully it's clear from expressions.md that you can use both methods

[LHeggy]

I think that was just testing out aliases. I've replaced it with something less confusing

[macebake]

This looks good and all works for me! You can merge and cut a minor release. Also it's probably worth following up w Hank from Netflix (you can find this on the original thread)