Promote Kotlin type-inferring APIs to the main Moshi package (round 2!) (#1202)

* Make moshi-root a kotlin project

* Move moshi kotlin extensions to moshi core

* Add appropriate experimental annotations

* Add nextAdapter helper

* Add explicit return type on addAdapter

* Expression body for adapter

* Use nextAdapter helper

* Opportunistically fix a couple Util warnings

* Add Types extensions

* Spotless

* Use extensions in more places for added coverage

* Apply java versions on any java plugin type

This way the kotlin projects get this too

* Fix circularAdapters test?

* Use java 8 in java for code gen too

* Fixup with CircularAdaptersTest

* Add coverage for remaining

* Remove nextAdapter

* Remove leftover function

* Use asserts

left checkNotNull for the contract

* boxIfPrimitive

* Fixup docs

* Copyright fixes

* Add parameterized addAdapter

* Switch to using native javaType API

* Spotless

* Back to 2019

* Spotless

* Use rawType extension

* Fix rebase issues

build.gradle.kts

@@ -120,6 +120,7 @@ subprojects {
     }
   }
 
+  // Apply with "java" instead of just "java-library" so kotlin projects get it too
   pluginManager.withPlugin("java") {
     configure<JavaPluginExtension> {
       sourceCompatibility = JavaVersion.VERSION_1_7