feat: add regex matching and substitution to stdlib

[zpeschke]

Add std.regexMatch and std.regexSubst functions into the standard library.

I simply ported these functions from Grafana Tanka's jsonnet extensions.

• https://tanka.dev/jsonnet/native/#regexmatch
• https://tanka.dev/jsonnet/native/#regexsubst
• https://github.com/grafana/tanka/blob/6a5c55958370b243d984156bfddf81cf35d95c93/pkg/jsonnet/native/funcs.go#L154-L180

Closes #409

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[ronniesena]

Eagerly awaiting for this to be merged! :) @zpeschke

[juice928]

👋 Hi, I'm an automated AI code review bot. I ran some checks on this PR and found 1 point that might be worth attention (could be false positives, please use your judgment):

1. The new regex builtins could benefit from regex compilation caching to improve efficiency.
   • Location: builtins.go:L1431-L1479
   • Impact: Repeatedly compiling the same pattern during loops or array operations may lead to high CPU overhead and slower performance.
   • Suggestion: Consider implementing a thread-safe cache (like an LRU cache or sync.Map) to store and reuse compiled regex objects.

If you find these suggestions disruptive, you can reply "stop" , and I'll automatically skip this repository in the future.