Automatically build JS/CSS assets of WP Rocket plugin on develop branch

[MathieuLamiot]

Context

This PR worked on running the gulp command automatically when releasing the plugin. Unfortunately, it was removed by mistake so it is not applied anymore. Additionally, we faced the issue within the team of forgetting to run the gulp command before testing a PR, making us lose time.

Expected behavior
Building JS and CSS assets for WP rocket plugin, currently manually handled with the gulp command should be automatically done.
There are several points to discuss to get the most efficient approach:

• When to trigger the action? On release seems too late in the process. On commit to develop? When updating a PR pointing to develop?
• How to leverage the action? As a reminder (run the minification process in the action and if the result is different, then fail the CI) or as an automated process (run the minification and commit)?