The great refactor

[lowlighter]

💣 This PR heavily refactor metrics code with a lot of meta-programming to simplify integration of new features:

• Plugins will now:
  • have their own README.md and documentation
  • have a metadata.yml file descriptor
    • containing possible inputs, along with types and default values
    • which simplifies all inputs checks (inputs from both web and action will be compliant with these metadata)
  • have their own GraphQL queries folder
    • which are still magically autoloaded like before
  • have base and core content, which were reclassified as "special" plugins
    • all misceallenous inputs (config_*, committer_*, debug, etc.) are now inputs of core plugin
• Templates will now:
  • have their own README.md and documentation
• Metrics README.md will now:
  • be autogenerated
    • by automatically extracting demo images from plugins and templates README.md
    • by automatically building compatibility matrix from present partials in each templates
    • by automatically indexing all templates
    • by automatically indexing all plugins
  • be splitted into smaller parts for easier maintaining
  • be more coherent in text style and more up-to-date
  • CONTRIBUTING.md has been rewritten to reflect this refactor
    • Also update issues and pull requests templates
• Metrics action.yml will now:
  • be autogenerated
    • by automatically extracting inputs from plugins metadata.yml files
• Metrics settings.example.json will now:
  • be autogenerated
    • by automatically extracting inputs from plugins metadata.yml files
• Web instance will now:
  • have plugins options extracted and autogenerated from metadata.yml plugins files
  • have app.placeholder.js being tested in workflow
• Mocks will now:
  • be splitted into smaller parts for easier maintaining
  • have mocked GitHub GraphQL API autoloaded
  • have mocked external REST APIs autoloaded
• Metadata system will now:
  • handle inputs of GitHub action
  • handle inputs of web instance
• Workflow will now:
  • automatically rebuild global metrics, plugins and templates README.md
  • automatically rebuild settings.example.json
  • automatically rebuild action.yml
  • automatically push the resulting files after run
• Relaunch lowlighter/lowlighter to check everything is fine