[FR]: Relative runfiles resolution

### What is the current behavior?

Every Bazel program eventually needs to look up data in runfiles. Currently this done in JavaScript by either:
* Accessing the `${RUNFILES}` environment variable like `${RUNFILES}/wksp/path/to/file.txt`.
* Using `@bazel/runfiles`. I think this technically works with `@aspect_rules_js`, though I've had trouble with it in the past.

### Describe the feature

It would be very cool to have an API which works with relative paths like other JavaScript features such as `import`. This can be achieved with `import.meta` like so:

```typescript
// wksp/path/to/script.mjs

import { promises as fs } from 'fs';
import { runfile } from '...';

// Returns `/.../wksp/path/to/file.txt`.
const filePath = runfile('./file.txt', import.meta);

const contents = await fs.readFile(filePath, 'utf8');
console.log(contents);
```

I'm not sure if `@aspect_rules_js` exposes any other JavaScript APIs, but I think this would be a useful and elegant one. It does require `import.meta` which necessitates either native ESM execution or a bundler which supports it. Exposing the absolute path to the runfile probably isn't ideal, since it leaks some information from outside the sandbox. Another approach would be to return a file path relative to the CWD, that way it could still be passed directly into APIs like `fs.readFile` and work as expected.

The actual implementation probably isn't too involved. [I did my own version of this pattern](https://github.com/dgp1130/rules_prerender/blob/3b6bbd4cfdf89aced79ce711e3e24845fb64a14a/packages/preact/index.mts#L94) with a Preact element ``. We could probably copy most of that implementation.

I can't think of any other JS APIs for `@aspect_rules_js` and I know from experience that pairing a Bazel ruleset with an NPM package can be quite challenging, so I can understand if this is out of scope. I just think it's a cool and elegant idea which is worth considering adopting directly into the ruleset.

Review @bazel/runfiles and the linked packages/preact/index.mts implementation first, then inspect how @aspect_rules_js currently exposes runfiles. Resolve the API shape, import.meta requirements, and sandbox-safe path behavior; done means the relative runfiles API is implemented and its proposed usage is covered by tests.

Written by the indexing model from the issue text.