Update org-twbs for org 9.8 compatibility

[jbrusey]

Summary

Added a reusable helper to generate outline-text containers directly from headlines, avoiding dependence on implicit section nodes during export.

Updated headline rendering to detect missing section elements and wrap their contents with the new helper so later headings render correctly under Org 9.8.

Added org-twbs-test-with-export helper macro and two ERT regression tests that assert later headlines and their outline containers are emitted in the TWBS exporter output.

Relaxed the headline assertions in org-twbs-export-includes-following-headlines so they tolerate the section-number spans added by Org during export.

[jbrusey]

@marsmining just want to check if I need to do anything further to this to get it reviewed.

[marsmining]

Thanks for the PR! I wanted to check the changes locally and found when trying to install from buffer I could not because of ;; Package-Requires: ((org "9.8")). Then I tried upgrading Org-mode as I was using 9.6.15 but on MELPA only 9.7.34 was available. Then I updated my Emacs from 29.4 to 30.2 and in that version Org-mode 9.7.11. Then I ran out of time :)

I'll try to find a bit of time in next few days. Sorry I've been slow.

[jbrusey]

Just for info, I'm on a mac using emacsplus under homebrew. M-x version gives GNU Emacs 31.0.50 (build 1, x86_64-apple-darwin23.6.0, NS appkit-2487.70 Version 14.6.1 (Build 23G93)) of 2025-02-26. M-x org-version gives Org mode version 9.8-pre (release_9.7.34-425-g526a7d