Allow + in MIDs as per MSC4009

[RosstheRoss]

This PR adds + to the username regex, per MSC4009.

Pull Request Checklist

• I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• Pull request includes a sign off below using a legally identifiable name or I have already signed off privately

Signed-off-by: Matt Strapp <[email protected]>

[RosstheRoss]

The original version of this PR had a bug where the regex was too loose and would allow some not allowed characters in a range, I added another test so no one else makes the same mistake I did 😅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d58daf9) 65.29% compared to head (022bb5c) 65.28%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3313      +/-   ##
==========================================
- Coverage   65.29%   65.28%   -0.01%     
==========================================
  Files         509      509              
  Lines       57520    57520              
==========================================
- Hits        37557    37552       -5     
- Misses      16058    16061       +3     
- Partials     3905     3907       +2     

Flag	Coverage Δ
unittests	50.04% <ø> (-0.02%)	⬇️

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[S7evinK]

LGTM, thanks for this!
One task down on #3226 :)