Cleanup coding standard

[Potherca]

This MR:

• Adds nextcloud/coding-standard as a development dependency
• Updates the existing PHP Codesniffer config to match the Nextcloud Coding Standard
• Fixes all PHP Codesniffer Violations

To review these changes, it is advised to toggle the "Ignore whitespace" setting, as the majority of the "fixes" are spaces ➡️ tabs. The few non-whitespace fixes are removing unused namespaces, and/or sorting the namespace in alphabetical order.

This MR has been marked as WIP because:

• Development documentation needs to be added
• The PHP code changes (and repercussions of those changes for existing developers/repos) need to be discussed/approved by existing @pdsinterop/main-team members.