…missing item (GromNaN)

This PR was merged into the 5.4 branch.

Discussion
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[Asset] [DX] Option to make asset manifests strict on missing item

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#14414

In all the projects I use a JSON manifest, when an asset is not listed in manifest.json, the asset file is not generated. The current behavior is permissive as it returns the unmodified path of the asset. Which ends with a 404 when the browser tries to load the asset.

With the option `strict_mode: true`, an exception is thrown when we try to use an asset that is not listed in `manifest.json`. Thereby we don't have to check that asset urls are actually working in tests (manual or automated).

**Usage:**
The option `strict_mode` is optional for backward compatibility. Using the `%kernel.debug%` value is safe to flush bugs on dev or test mode but keep the application working on production.

```yaml
# config/packages/assets.yaml

framework:
    assets:
        packages:
            app:
                # Uses a JSON manifest (can be a local path or an url remote file)
                json_manifest_path: '%kernel.project_dir%/public/build/manifest.json'
                # Throws an exception when an expected entry is missing in the manifest
                strict_mode: '%kernel.debug%'

```

Todo:
- [x] Documentation symfony/symfony-docs#14414
- [x] Demo symfony/demo#1168
- [x] ~Update recipe ?~

Commits
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4cdb921 Option to make asset manifests strict on missing item