Extract expression resolver into a standalone OSS package

**Parent Issue:** #71

## Summary

The expression resolver implementation in the CLI (handling `$(env:...)`, `$(file:...)`, etc.) is generic enough that it could be extracted into a standalone open-source package. This would let other CLI tools and config systems reuse the same resolution logic.

## Motivation

Suggested by @JoshMock in #143. The resolver is self-contained, well-tested, and solves a common problem (resolving dynamic expressions in YAML/JSON config). Publishing it as a standalone package would benefit the broader ecosystem.

## Scope

- Extract the resolver into its own package (e.g., `@elastic/config-resolver` or similar)
- Publish to npm
- Maintain the CLI's dependency on the extracted package