Support global-only declaration packages such as @webgpu/types

## Problem

`@webgpu/types` is a type-only package whose `dist/index.d.ts` declares WebGPU browser globals such as `GPUDevice`, `GPUBuffer`, `GPUTexture`, and additions to `Navigator`/canvas interfaces. It is not an importable JavaScript module.

After fixing the Windows scratch-cache path in #193, the package installs and its declaration entry resolves correctly, but extraction stops at the module boundary:

```text
$ npx @juspay/rescript-bindgen --pkg @webgpu/types --out webgpu-types
[bindgen] entry: .../.bindgen-cache/node_modules/@webgpu/types/dist/index.d.ts
[bindgen] @module("@webgpu/types")
[bindgen] error: No module symbol for .../@webgpu/types/dist/index.d.ts
```

Tested with `@webgpu/types@0.1.71`.

## Root cause

The extractor currently starts from either:

- a source file with top-level module exports; or
- an ambient `declare module "..."` block.

It then emits React components, exported functions, and exported classes, with runtime access generally rooted in `@module(...)`.

A global-script declaration file has neither kind of module symbol. Even if it were allowed past that check, `@module("@webgpu/types")` would be incorrect because the package has no runtime module to import, and top-level type/interface declarations are not currently treated as generation roots.

## Proposed support

Add a global-declarations extraction/emission mode for global-only `.d.ts` packages:

- detect an entry containing global declarations but no external/ambient module symbol;
- traverse relevant top-level global interfaces, type aliases, variables, constructors, and functions;
- emit browser-global runtime bindings with the appropriate `@val`, `@new`, `@get`, `@set`, and `@send` annotations instead of `@module`;
- make referenced type aliases and interfaces available as generated ReScript types;
- preserve declaration merging/augmentations such as `Navigator`, `HTMLCanvasElement`, and `OffscreenCanvas` additions;
- report unsupported global declarations explicitly rather than silently dropping them.

Whether this is auto-detected or enabled through an explicit flag can be decided during implementation, but normal importable-package behavior must remain unchanged.

## Acceptance criteria

- `@webgpu/types` gets past resolution and extraction without `No module symbol`.
- Generated bindings never attempt to import a runtime `@webgpu/types` module.
- A representative slice of WebGPU globals is usable from ReScript and compiles under the supported ReScript version.
- Global constructors/constants and instance methods/properties use correct interop annotations.
- A self-contained global-only golden fixture covers the mode without requiring network access.
- Existing module and ambient-module fixtures remain byte-identical unless intentionally updated.

## Related

- #193 fixes the separate Windows `C:\\C:\\...` scratch-cache path problem exposed before extraction begins.

Start at the extractor's module-symbol resolution path, where the global-only declaration currently fails, and compare the existing module and ambient-module fixture behavior. Use a self-contained global-only golden fixture to cover representative WebGPU declarations without network access. Done means extraction succeeds without importing @webgpu/types, generated bindings compile under the supported ReScript version, and existing fixtures remain unchanged.

Written by the indexing model from the issue text.