rx (feature): Add Rx.mapToRx as an alias of flatMap

[xerial]

No description provided.

[codecov]

Codecov Report

Merging #3135 (0161408) into main (24304d0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3135   +/-   ##
=======================================
  Coverage   82.88%   82.88%           
=======================================
  Files         350      350           
  Lines       14736    14737    +1     
  Branches     2440     2440           
=======================================
+ Hits        12214    12215    +1     
  Misses       2522     2522           

Files Changed	Coverage Δ
...frame-rx/src/main/scala/wvlet/airframe/rx/Rx.scala	83.00% <100.00%> (+0.17%)	⬆️

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