update composer dependencies

- add .editorconfig file
- format all files to comply with castopod's coding style
- switch parsedown dependency with commonmark library to better follow commonmark spec for markdown
- add prettier command to format all project files at once

closes #16

- add james-heinrich/getid3 library as a dependency to composer.json
- update DEPENDENCIES.md file
- fix episodes table migration script
- add js devDependencies: prettier, @prettier/plugin-php and lint-staged to automatically format staged files before commit
- reformat all files to the prettier format
- refactor code by separating some logic as helper functions
- overwrite existing files when uploading new files with the same name

fixes #1

- add node service in docker-compose.yml
- update .devcontainer Dockerfile by adding node, npm and vim
- init package.json for npm with tailwindcss, postcss, commitlint and commitizen as dev dependencies
- update default layout main header and footer
- replace CI's welcome_message.php with home.php listing all podcasts
- add AUTHORS.md file
- add docs folder in which to place castopod's technical documentation