feat: merge https://github.com/ensan-hcl/CustardExpressionEvaluator and modernize repository structure

Author: ensan-hcl

Package.resolved

@@ -0,0 +1,14 @@
+{
+  "pins" : [
+    {
+      "identity" : "swift-argument-parser",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/apple/swift-argument-parser",
+      "state" : {
+        "revision" : "41982a3656a71c768319979febd796c6fd111d5c",
+        "version" : "1.5.0"
+      }
+    }
+  ],
+  "version" : 2
+}

Package.swift

@@ -10,11 +10,14 @@ let package = Package(
         .library(
             name: "CustardKit",
             targets: ["CustardKit"]
+        ),
+        .executable(
+            name: "samples",
+            targets: ["samples"]
         )
     ],
     dependencies: [
-        // Dependencies declare other packages that this package depends on.
-        // .package(url: /* package url */, from: "1.0.0"),
+        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.3.0"),
     ],
     targets: [
         // Targets are the basic building blocks of a package. A target can define a module or a test suite.
@@ -28,6 +31,14 @@ let package = Package(
             name: "CustardKitTests",
             dependencies: ["CustardKit"],
             path: "swift/tests/CustardKitTests"
+        ),
+        .executableTarget(
+            name: "samples",
+            dependencies: [
+                "CustardKit",
+                .product(name: "ArgumentParser", package: "swift-argument-parser"),
+            ],
+            path: "swift/examples"
         )
     ]
 )