Extract sbt-airframe as a separate GitHub project

[xerial]

This PR extracts sbt-airframe module as a separate GitHub project https://github.com/wvlet/sbt-airframe.

This is for simplifying the cross build settings.
Currently, sbt-plugin must use only Scala 2.12, while the other airframe modules can be built with Scala 2.13 and Scala 3.
Having a sbt-plugin inside the same project has caused a lot of dependency errors when switching to Scala 2.13 or Scala 3.

With this change, we can make the default Scala version of Airframe at least to Scala 2.13. And also, cross building Scala.js projects with Scala 3 will be possible.

[codecov]

Codecov Report

Merging #1686 (c186a69) into master (7242187) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1686   +/-   ##
=======================================
  Coverage   82.77%   82.77%           
=======================================
  Files         310      310           
  Lines       12253    12253           
  Branches      753      753           
=======================================
+ Hits        10142    10143    +1     
+ Misses       2111     2110    -1     

Impacted Files	Coverage Δ
...rx/src/main/scala/wvlet/airframe/rx/RxRunner.scala	91.22% <0.00%> (+0.43%)	⬆️

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[xerial]

cc: @wvlet/airframe-dev