Update for Flarum 2.x, use flexbox layout and add height parameters

[dsevillamartin]

Changes proposed in this pull request:

• Update composer.json required versions and phpstan config for Flarum 2.x
• Update JS for Flarum 2.x and switch to yarn
• Switch float layout to flexbox with .content using 100% width and appearing last
• Adjust CSS to only affect direct children to avoid potential issues
• Improve tab IDs system to increment ids only within posts (or globally if not related to a post). ID adjusted to be tab-post<ID>-tabs<NUM>
• Add fullheight and height BBCode attributes for [tabs] and [tab] (tab overrides parent tabs) using CSS variables to allow adjusting content height. fullheight makes it auto so it just adapts to contents, and changes depending on content length
• Add support for tabs in FoF Pages

Reviewers should focus on:

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).