✨ add #tag function

[lostdesign]

While the upcoming search may not need this, some may want to explicitly set their tags themselves. flexsearch does support tags and storing additional values would not hurt anyways.

Acceptance Criteria

• use tiptap in order to create a custom node when someone types a # followed by some text
• that new extension then should push the tag into the current day file
• flexsearch should be extended by the tags attribute, which is just tags itself, from there on, it would register the tags
• deleting tags should remove them from the day file as well as the search index (maybe the later is automatic, to be tested)
• adding duplicates should not be possible