feat(jinja): webpack_optional global helper

[mirekys]

This allows for optional inclusion of webpack manifest:

{{ webpack_optional('myentry') }}

If requested manifest not found in manifest.json, no hard error is raised.
Instead, error gets just output to resulting markup as a plain comment:

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Ducica]

@mirekys It looks good to me, after we should just make sure to remove all the current overridable registry implementation including the entry point.