chore: Add list_api script (#3341)

<!-- .github/pull_request_template.md -->

## 📌 Description

A utility script that can be used for API review/QA purposes

## 🔍 Related Issues

<!-- Link any related issues here -->

## 🚀 Pull Request Checklist

Thank you for contributing to FlashInfer! Before we review your pull
request, please make sure the following items are complete.

### ✅ Pre-commit Checks

- [x] I have installed `pre-commit` by running `pip install pre-commit`
(or used your preferred method).
- [x] I have installed the hooks with `pre-commit install`.
- [x] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

> If you are unsure about how to set up `pre-commit`, see [the
pre-commit documentation](https://pre-commit.com/).

## 🧪 Tests

- [x] Tests have been added or updated as needed.
- [x] All tests are passing (`unittest`, etc.).

## Reviewer Notes

**Examples**

List all `@flashinfer_api`-decorated APIs (module-level and class
methods), grouped by class (or `[Global Functions]` per file), with full
multi-line signatures.

### List current API surface
```bash
scripts/list_apis.sh
```

### Signatures only (no paths, no line numbers)
```bash
scripts/list_apis.sh -p
```

### Class methods only (skip module-level functions)
```bash
scripts/list_apis.sh -M
```

### Inspect a single file
```bash
scripts/list_apis.sh -p flashinfer/cascade.py
```

### Run against a git tag / branch / SHA
Auto-fetches from `upstream` or `origin` if not present locally; uses a
throwaway worktree.
```bash
scripts/list_apis.sh --ref v0.6.9
scripts/list_apis.sh --ref main -p
```

### Diff the API surface between two revisions
Use `-d` for stable, byte-identical output so the diff only reflects
real API changes.
```bash
diff -u \
  <(scripts/list_apis.sh -d -p --ref v0.6.9) \
  <(scripts/list_apis.sh -d -p)
```

### Flags
| Flag | Effect |
|---|---|
| `-n`, `--no-lines`        | Omit line numbers |
| `-p`, `--no-paths` | Omit paths *and* line numbers (signatures-only) |
| `-M`, `--methods-only` | Skip module-level functions; only show class
methods |
| `-d`, `--deterministic` | Stable, diff-friendly output (sorts files;
slightly slower) |
| `-r`, `--ref REF` | Run against a git revision via temp worktree |
| `-h`, `--help`            | Show help |


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Added a command-line utility to discover and list decorated Python
APIs across the codebase.
* Preserves full multi-line function signatures and offers output
options to hide line numbers or file paths.
* Can limit results to class methods and supports scanning a specific
repository revision via a temporary checkout.
  * Includes an option for stable, deterministic ordering of results.

<!-- review_stack_entry_start -->

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/flashinfer-ai/flashinfer/pull/3341?utm_source=github_walkthrough&utm_medium=github&utm_campaign=change_stack)

<!-- review_stack_entry_end -->
<!-- end of auto-generated comment: release notes by coderabbit.ai -->