Add a JavaKit sample that produces a Swift command-line app using Java APIs (#79)

* Remove stale JavaKitExample sources

* Add a JavaKit sample that produces a Swift command-line app using Java APIs

This demonstrates how to pull in just one Java type (java.math.BigInteger)
and use it from a Swift program (a command line app).

Author: DougGregor

Samples/JavaProbablyPrime/Package.swift

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+// swift-tools-version: 6.0
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import CompilerPluginSupport
+import PackageDescription
+
+let package = Package(
+  name: "JavaProbablyPrime",
+  platforms: [
+    .macOS(.v13),
+    .iOS(.v13),
+    .tvOS(.v13),
+    .watchOS(.v6),
+    .macCatalyst(.v13),
+  ],
+
+  products: [
+    .executable(
+      name: "JavaProbablyPrime",
+      targets: ["JavaProbablyPrime"]
+    ),
+  ],
+
+  dependencies: [
+    .package(name: "swift-java", path: "../../"),
+    .package(url: "https://github.com/apple/swift-argument-parser", from: "1.5.0"),
+  ],
+
+  targets: [
+    .executableTarget(
+      name: "JavaProbablyPrime",
+      dependencies: [
+        .product(name: "JavaKit", package: "swift-java"),
+        .product(name: "JavaKitVM", package: "swift-java"),
+        .product(name: "ArgumentParser", package: "swift-argument-parser"),
+      ],
+      swiftSettings: [
+        .swiftLanguageMode(.v5)
+      ],
+      plugins: [
+        .plugin(name: "Java2SwiftPlugin", package: "swift-java"),
+      ]
+    ),
+  ]
+)

Samples/JavaProbablyPrime/README.md

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+# JavaKit Example: Using Java APIs from Swift
+
+This package contains an example program that uses Java's [`java.math.BigInteger`](https://docs.oracle.com/javase/8/docs/api/?java/math/BigInteger.html) from Swift to determine whether a given number is probably prime. You can try it out with your own very big number:
+
+```
+swift run JavaProbablyPrime <very big number>
+```
+
+The package itself demonstrates how to:
+
+* Use the Java2Swift build tool plugin to wrap the `java.math.BigInteger` type in Swift.
+* Create a `JavaVirtualMachine` instance in a Swift command-line program.
+* Create an instance of `BigInteger` in Swift and use its `isProbablyPrime`.

Samples/JavaProbablyPrime/Sources/JavaProbablyPrime/Java2Swift.config

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+{
+  "classes" : {
+    "java.math.BigInteger" : "BigInteger"
+  }
+}

Samples/JavaProbablyPrime/Sources/JavaProbablyPrime/prime.swift

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import ArgumentParser
+import JavaKit
+import JavaKitVM
+
+@main
+struct ProbablyPrime: ParsableCommand {
+  @Argument(help: "The number to check for primality")
+  var number: String
+
+  @Option(help: "The certainty to require in the prime check")
+  var certainty: Int32 = 10
+
+  func run() throws {
+    let javaVirtualMachine = try JavaVirtualMachine.shared()
+    let jniEnvironment = try javaVirtualMachine.environment()
+
+    let bigInt = BigInteger(number, environment: jniEnvironment)
+    if bigInt.isProbablePrime(certainty) {
+      print("\(number) is probably prime")
+    } else {
+      print("\(number) is definitely not prime")
+    }
+  }
+}