feat(timeline): Use read receipt as fallback for read marker

[VAWVAW]

According to the spec if a m.fully_read event does not exist, the normal m.read marker should be used as a fallback.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.44%. Comparing base (588d604) to head (de65c94).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5772      +/-   ##
==========================================
+ Coverage   88.43%   88.44%   +0.01%     
==========================================
  Files         360      360              
  Lines       99796    99805       +9     
  Branches    99796    99805       +9     
==========================================
+ Hits        88250    88273      +23     
+ Misses       7405     7395      -10     
+ Partials     4141     4137       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[codspeed-hq]

CodSpeed Performance Report

Merging #5772 will not alter performance

_{Comparing VAWVAW:timeline-fully-read-fallback (de65c94) with main (588d604)}

Summary

✅ 50 untouched