EZP-22767: ezscript_require not taken into account on modules

Link: https://jira.ez.no/browse/EZP-22767

## Description
ezscript_require (legacy) not taken into account on modules. 

This PR adds two new twig operators `ez_legacy_render_js()` and `ez_legacy_render_css()`.

They are needed when using legacy templates with ezjscore requirements in module.
Including these operator in the twig template in charge of  the `<head>` of your page will generate the HTML code needed to include js and css.

Example of usage in the demobundle: https://github.com/ezsystems/DemoBundle/pull/89

## Tests
Manual and unit tests.