Add support for systemLibrary and executableTarget types

[Copilot]

This PR extends the PackageGeneratorPlugin to support all Swift Package Manager target types, addressing the limitation where only .target and .testTarget were previously supported.

Changes Made

Core Implementation

• Added TargetType enum with support for all five target types: target, testTarget, executableTarget, systemLibrary, and binaryTarget
• Enhanced ParsedPackage struct to include a targetType field with backward-compatible auto-detection based on the existing isTest flag
• Updated target generation logic in fakeTargetToSwiftCode to generate appropriate Swift Package Manager target declarations

Smart Detection & Configuration

• Auto-detection for executable targets: Automatically detects executable targets by checking for the presence of a main.swift file
• Configuration override support: Added targetTypes configuration field allowing manual specification of target types for specific targets
• Enhanced product generation: Generates .executable products for executable targets and .library products for regular targets, while excluding system libraries and binary targets from product generation

Validation & Error Handling

• Configuration validation: Warns users about invalid target type values and unused target type configurations
• Comprehensive error checking: Validates that specified target types are valid enum values

Documentation & Examples

Updated README with comprehensive documentation including:

• Description of the new targetTypes configuration option
• Example configuration showing how to specify different target types
• Clear explanation of valid target type values

Example Usage

{
  "targetTypes": {
    "MyApp": "executableTarget",
    "SystemWrapper": "systemLibrary", 
    "PrecompiledFramework": "binaryTarget"
  },
  "targetsParameters": {
    "SystemWrapper": [
      "pkgConfig: \"libxml-2.0\"",
      "providers: [.brew([\"libxml2\"]), .apt([\"libxml2-dev\"])]"
    ]
  }
}

This generates appropriate Swift Package Manager declarations:

.executableTarget(name: "MyApp", dependencies: [...], path: "..."),
.systemLibrary(name: "SystemWrapper", path: "...", pkgConfig: "libxml-2.0", ...),
.binaryTarget(name: "PrecompiledFramework", path: "...")

Backward Compatibility

All existing configurations continue to work without modification. The implementation uses sensible defaults and maintains the existing behavior for projects that don't specify custom target types.

Fixes #8.

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