Publish executables for pkldoc, pkl-codegen-java, pkl-codegen-kotlin (#1023)

This adds logic to build and publish the other executables related to Pkl.

These are:

* pkl-doc
* pkl-codegen-kotlin
* pkl-codegen-java

pkl-codegen-kotlin and pkl-codegen-java are published as executable JARs, whereas pkldoc is published both as an executable JAR, and also native executables (matching the set of os/arch supported by Pkl).

The reason this only publishes executable JARs for pkl-codegen-kotlin and pkl-codegen-java is because we expect that the Java requirement is not a problem for these users, and that the native executable provides negligible added value.

As part of this, the following changes are made:

* Introduce `pklJavaExecutable` plugin, which sets up building and publishing of executable JAR.
* Introduce `pklNativeExecutable` plugin, which sets up building and publishing of native executables.
* Introduce `NativeImageBuild` Gradle task, which knows how to build native-image executables.
* Introduce `ExecutableSpec` extension, for projects that publish executables to configure how those executables should be published.
* `./griddles buildNative`, by default, will only build the executable of the host OS/Arch, and will no longer cross-build.
* The target arch of `./gradlew buildNative` can be changed using `-Dpkl.targetArch=<aarch64|amd64>`.
* On linux/amd64 only, with `./gradlew buildNative`, a statically linked executable can be built using `-Dpkl.musl=true`
* Make `javaExecutable` a dependency of `assemble`
* Make `testStartJavaExecutable` a dependency of `check`
* Change name `pklNativeBuild` to `pklNativeLifecycle` to better match the plugin's purpose
* Remove Truffle SVM classes from main source set (don't publish these classes as part of the pkl-cli JAR)
* Change CircleCI definition to publish new executables
* Change CircleCI definition to call `buildNative` instead of individual task names

.circleci/jobs/DeployJob.pkl

@@ -30,11 +30,14 @@ steps {
   new Config.RunStep {
     command = "./gradlew \(self.gradleArgs) \(module.command)"
   }
-  // add jpkl to workspace so it gets published as a GitHub release
+  // add Java executables to workspace so they get published as a GitHub release
   new Config.PersistToWorkspaceStep {
     root = "."
     paths {
       "pkl-cli/build/executable/"
+      "pkl-doc/build/executable/"
+      "pkl-codegen-java/build/executable/"
+      "pkl-codegen-kotlin/build/executable/"
     }
   }
 }