Add CancelClosingModalWhenFocusLost extension method.

[maliming]

<Modal @ref="MyModal" Closing="MyModal.CancelClosingModalWhenFocusLost">

[codecov]

Codecov Report

Merging #13317 (b2dd904) into rel-5.3 (80e96ea) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           rel-5.3   #13317   +/-   ##
========================================
  Coverage    49.44%   49.44%           
========================================
  Files         3117     3117           
  Lines        93245    93245           
========================================
  Hits         46103    46103           
  Misses       47142    47142           

Impacted Files	Coverage Δ
...ame.MongoDB/MongoDb/MyProjectNameMongoDbContext.cs	0.00% <ø> (ø)

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