Invert `regexp` operator

### Proposal
I want to have a constraint that makes the job **avoid** clients with `batch` (substring) in their metadata `tags` (string) field.
The issue is loosely related to https://github.com/hashicorp/nomad/issues/6610.

`test, batch, demo` --> avoid
`test, demo` --> fine

### Attempted Solutions

1. I've tried this:
```hcl
constraint {
attribute = "${meta.tags}"
operator = "regexp"
value = "^(?!.*batch).*$"
}
```
But it looks the Go engine does not support negative lookaheads for performance reasons ([ref](https://stackoverflow.com/questions/26771592/negative-look-ahead-in-go-regular-expressions)).

2. One option would be to negate the `regexp` operator, but `regexp_not` does not seem to exist in Nomad.

Is there any way to achieve this in Nomad? I've ended up moving `batch` to a different individual metadata field, but I would like to know if it's possible just in case for next time.

Thanks!

No source file or test is named. Start by tracing Nomad’s existing `regexp` constraint operator and its Go regular-expression handling; done should support an inverse form that avoids clients whose `meta.tags` contains `batch`, while preserving the shown matching behavior.

Written by the indexing model from the issue text.