Build AirSpec using a sub project

[xerial]

This is for simplifying build configuration as IntelliJ IDEA cannot properly scan source code files referenced multiple times in the same projects.

• Split AirSpec as a sub project within the same GitHub repository
• Use libraryDependency for AirSpec
• Tell @SethTisue that we need to change the build configuration of the scala community build to build AirSpec for Scala JVM like this:

cd airspec
sbt +airspecJVM/publishLocal

[codecov]

Codecov Report

Merging #1992 (bb06232) into master (9d96fc1) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1992      +/-   ##
==========================================
- Coverage   82.64%   82.52%   -0.13%     
==========================================
  Files         317      301      -16     
  Lines       12166    11828     -338     
  Branches      770      748      -22     
==========================================
- Hits        10055     9761     -294     
+ Misses       2111     2067      -44     

Impacted Files	Coverage Δ
airspec/src/main/scala/wvlet/airspec/AirSpec.scala
...ain/scala/wvlet/airspec/spi/AirSpecException.scala
...spec/src/main/scala/wvlet/airspec/AirSpecDef.scala
.../main/scala/wvlet/airspec/runner/AirSpecTask.scala
...rspec/src/main/scala/wvlet/airspec/CompatApi.scala
...c/main/scala/wvlet/airspec/spi/PropertyCheck.scala
...src/main/scala/wvlet/airspec/spi/RichAsserts.scala
.../scala/wvlet/airspec/runner/AirSpecSbtRunner.scala
...scala/wvlet/airspec/runner/AirSpecTaskRunner.scala
...pec/src/main/scala/wvlet/airspec/spi/Asserts.scala
... and 6 more

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