Provide preset-like factory functions for creating targets in new MPP

* Pull the `targets` and `presets` properties of the `kotlin` extension
  up to an interface in `kotlin-gradle-plugin-api`

* Generate the code: we need type-safe statically-typed functions for
  different target type, which seems to be only possible to achieve by
  providing a function per preset.

* Place these functions in the top-level `kotlin` Gradle extension,
  rather than extension functions for `kotlin.targets`:

  - `kotlin.jvm { ... }` will work, while `kotlin.targets.jvm { ... }`
    won't, the extension function needs to be somehow brought into scope

  - reducing the nesting level by one seems to be a good move here

Issue #KT-26389 Fixed

settings.gradle

@@ -157,6 +157,7 @@ include ":kotlin-build-common",
         ":tools:kotlin-stdlib-gen",
         ":tools:kotlinp",
         ":kotlin-gradle-plugin-api",
+        ":kotlin-gradle-plugin-dsl-codegen",
         ":kotlin-gradle-plugin",
         ":kotlin-gradle-plugin-model",
         ":kotlin-gradle-plugin-test-utils-embeddable",
@@ -293,6 +294,7 @@ project(':tools:kotlin-stdlib-js-merger').projectDir = "$rootDir/libraries/tools
 project(':tools:kotlin-stdlib-gen').projectDir = "$rootDir/libraries/tools/kotlin-stdlib-gen" as File
 project(':tools:kotlinp').projectDir = "$rootDir/libraries/tools/kotlinp" as File
 project(':kotlin-gradle-plugin-api').projectDir = "$rootDir/libraries/tools/kotlin-gradle-plugin-api" as File
+project(':kotlin-gradle-plugin-dsl-codegen').projectDir = "$rootDir/libraries/tools/kotlin-gradle-plugin-dsl-codegen" as File
 project(':kotlin-gradle-plugin').projectDir = "$rootDir/libraries/tools/kotlin-gradle-plugin" as File
 project(':kotlin-gradle-plugin-model').projectDir = "$rootDir/libraries/tools/kotlin-gradle-plugin-model" as File
 project(':kotlin-gradle-plugin-test-utils-embeddable').projectDir = "$rootDir/libraries/tools/kotlin-gradle-plugin-test-utils-embeddable" as File