Proposal: Unify bundler plugins using unplugin

Nobody has claimed this yet.

### Problem Description

Lingui maintains separate packages for different bundlers:

- `@lingui/vite-plugin` - Vite plugin
- `@lingui/loader` - Webpack loader

Both use shared logic from `@lingui/cli/api`, but require separate packages and documentation. Adding support for new bundlers (esbuild, etc.) would mean creating and maintaining more packages. Tests are also duplicated for each bundler.

### Proposed Solution

Use [unplugin](https://github.com/unjs/unplugin) to create a unified plugin that works across multiple bundlers from a single codebase.

### Package structure

New package: `@lingui/plugin`

```
@lingui/plugin
├── /vite - Vite plugin
├── /webpack - Webpack plugin
├── /esbuild - esbuild plugin
└── /rspack - Rspack plugin
```

Usage:
```ts
// vite.config.ts
import { defineConfig } from 'vite'
import { lingui } from '@lingui/plugin/vite'

export default defineConfig({
plugins: [lingui()],
})
```

```ts
// webpack.config.ts
import { lingui } from '@lingui/plugin/webpack'

export default {
plugins: [lingui()],

// No need to setup loader manually, the plugin does it for you
// module: {
// rules: [
// {
// test: /\.po$/,
// use: '@lingui/loader',
// },
// ],
// },
}
```

### Reference

TanStack Router uses this approach:
- Package: `@tanstack/router-plugin`
- Source: https://github.com/TanStack/router/tree/main/packages/router-plugin

### Migration path

1. Create `@lingui/plugin` package
2. Deprecate `@lingui/vite-plugin` and `@lingui/loader`
3. Remove old packages in a future major version

### Notes

- Metro (React Native) is not supported by unplugin yet, so `@lingui/metro-transformer` remains separate

### Alternatives Considered

Keep `@lingui/vite-plugin` and `@lingui/loader` as separate packages, adding new packages for each bundler.

**Drawbacks:**
- More packages to maintain
- Duplicated boilerplate code
- Harder to ensure consistent behavior across bundlers

Review the existing @lingui/vite-plugin and @lingui/loader packages, their shared @lingui/cli/api logic, and the TanStack Router plugin reference. Define the @lingui/plugin package entry points for Vite, Webpack, esbuild, and Rspack, while keeping @lingui/metro-transformer separate. Done means the migration path, deprecations, and unified plugin behavior are established.

Written by the indexing model from the issue text.