fix(ui): fix replaceMentions not working with usernames.

[xsahil03x]

Fixes: #1702

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09% 🎉

Comparison is base (fb9ab5a) 58.19% compared to head (1264fa1) 58.28%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1706      +/-   ##
===========================================
+ Coverage    58.19%   58.28%   +0.09%     
===========================================
  Files          292      292              
  Lines        17975    17975              
===========================================
+ Hits         10460    10477      +17     
+ Misses        7515     7498      -17     

Files Changed	Coverage Δ
.../stream_chat_flutter/lib/src/utils/extensions.dart	31.25% <100.00%> (+8.17%)	⬆️

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