feat(cli): Experimental config for swift-tools-version in SPM apps

[OS-pedrogustavobilro]

PR Description

⚠️ AI-assisted PR with Claude, manually reviewed and verified by me.

This PR adds a new experimental for setting swift-tools-version in the CapApp-SPM/Package.swift file. The reason why this feature is marked experimental is because Capacitor does not support Swift 6 yet, and therefore using this config to set swift-tools-version to 6.x can cause issues for their apps, depending on the plugins the app has, their dependencies, and what Swift version they were built with.

Usage example in capacitor.config.json:

{
	// other configs...
	"experimental": {
		"ios": {
			"spm": {
				"swiftToolsVersion": "6.1"
			}
		}
	}
}

Related to #8351, as a way for users to make a more conscious opt-in for Package Traits in SPM.

Testing

I've tested setting SwiftToolsVersion '6.1', and also '6.2.1' (latest in current stable Xcode version) with most official Capacitor plugins across different apps, and the apps build and plugins working without issues.

Feel free to create your own apps, point to the local version of capacitor (cli) containing this PR branch, add the experimental config, run cap sync, build the app with Xcode and test.