maintenance/update-developer-experience: update structure, docs and developer experience

[vi-h]

Based on changes from branch feature/app-lib (which is now removed).

• Use Makefile, and es6 for development.
• Updated Readme to reflect changes
• Updated Readme to add more information about the headers
• Updated Gradle version and handling
• Added linting

[Roeland-Bouvet]

Builds fine locally, and looks good when installed locally. But have a few small remarks:

• Noticed there is both a response filter and processor with identical code. However, the filter is commented out in site.xml, so perhaps we can just delete it entirely? 🤔
• The commit hash will now be added to the version number with .${grgit.head().abbreviatedId}". Is this intentional? I know it's done in other projects, but personally not a big fan of it, though no strong feelings. If you prefer it, it's fine for me. 😅
• I ran a build of this branch in jitpack and it failed ( https://jitpack.io/com/github/bouvet-apps/security-headers/maintenance~update-developer-experience-ba16858ba6-1/build.log ). The error is one we've encountered before, and is due to the node version. So might need to downgrade node version again. Also mentioned it here https://github.com/bouvet/team-enonic-commons/blob/main/docs/en/publish-to-enonic-market.md#known-build-issues