[Feature] Add Swift Package Manager support

[OhKanghoon]

This adds a Package.swift so PrimJS can be consumed with Swift Package Manager in addition to CocoaPods.

What's in it

The manifest is a 1:1 mapping of PrimJS.podspec. Sources, preprocessor definitions and header search paths are taken from the podspec and point at the existing src/ tree; nothing is copied or wrapped. Sources are declared per directory wherever the podspec uses a recursive glob; the few files listed individually (napi.cc, napi_module.cc, the two adapter files) are the ones the podspec also lists individually, with the reason next to each.

CocoaPods subspec	SwiftPM
PrimJS/quickjs + PrimJS/log	PrimJS
PrimJS/quickjs_debugger	Debugger package trait (internal PrimJSInspector target)
PrimJS/napi/{core,env,quickjs}	PrimJSNAPI
PrimJS/napi/jsc	PrimJSNAPIJSC
PrimJS/napi/adapter	PrimJSNAPIAdapter

Public headers. SwiftPM allows a single public header directory per target and cannot rename or re-root headers, while CocoaPods assembles Headers/Public/PrimJS/<header_dir>/<name> from public_header_files, header_dir and header_mappings_dir. tools/ios_tools/generate_spm_headers.py writes that same layout as relative symlinks under swiftpm/headers/<target>/, so the include paths are identical for both package managers (quickjs/include/quickjs.h, quickjs/include/trace-gc.h, napi_env_quickjs.h, ...). I checked the generated tree against what pod install produces for the same subspecs; all 65 entries match. The script is plain Python with a declarative table of what each target publishes and has no dependency on CocoaPods, so the SwiftPM setup keeps working after the podspec is gone. Its --check mode runs in CI and fails with the list of stale/missing entries when a public header is added or moved without regenerating.

Debugger. quickjs_debugger is a package trait instead of a product. Adding the subspec to a pod changes ENABLE_QUICKJS_DEBUGGER for the whole pod target, which changes struct layouts in quickjs-inner.h and switches the snapshot to embedded-inspector.S. A separate SwiftPM target layered on top of the plain engine would therefore link the wrong snapshot and see the wrong ABI; the trait reproduces the pod behaviour. This is the reason for swift-tools-version: 6.1 (Xcode 16.3+).

Swift. Without a module map SwiftPM treats the whole public header directory as the module's umbrella, so import PrimJS would fail on the first C++ header (the same happens with CocoaPods use_frameworks!, whose umbrella imports every public header). A hand-written umbrella header in swiftpm/headers/PrimJS/ lists the C API (quickjs.h, preceded by <stdbool.h> since the header is written for C++) and a three-line module map points at it, so Swift can call the engine directly. Objective-C++ consumers are unaffected; they never go through the module. The header generator only manages the symlinks and leaves these two files alone.

Tests. swift build only archives the library targets, so a small XCTest target (one .mm file) links the engine, evaluates a script on the template interpreter and runs one through the Node-API layer; a second one does import PrimJS from Swift. Both run with and without the trait.

CI. A swift-package job runs the header check, swift build/swift test on macOS with and without Debugger, xcodebuild test on the iOS Simulator and a generic/platform=iOS build. It uses macos-latest; happy to move it to the self-hosted darwin runners if that's preferred.

Limitations

• arm64 only, same as the podspec: the template interpreter ships as pre-generated arm64 assembly, so x86_64 simulator builds don't link.
• As with CocoaPods, consumers of the Node-API headers have to define ENABLE_CODECACHE themselves; SwiftPM can't propagate defines to dependents. Noted in Package.swift next to the test target, which does exactly that.

Verification

• swift build, swift test, both with --traits Debugger as well (macOS arm64, Xcode 26.4)
• xcodebuild test -scheme PrimJS-Package on an iOS 27 simulator, xcodebuild build -destination generic/platform=iOS
• python3 tools/ios_tools/generate_spm_headers.py --check
• Fresh clone of the branch builds and tests from the committed files alone
• pod spec lint is unaffected: no existing file is modified except .gitignore and ci.yml

[cla-assistant]

All committers have signed the CLA.