feat: tvOS support for url_launcher

[TheNoumanDev]

What does this PR do?

Adds url_launcher_tvos, the federated tvOS implementation of url_launcher, ported from url_launcher_ios 6.4.1. canLaunchUrl and external launchUrl work on tvOS via UIApplication.canOpenURL / open(_:options:); the in-app browser modes rely on SFSafariViewController (SafariServices), which does not exist on tvOS, so those are reported unsupported rather than crashing.

Closes #16.

Package(s) touched: url_launcher_tvos (new) + root README ports table.

How was it tested?

• Ran the package's example/ app

• Verified on tvOS simulator (version: 26.2)

• Verified on a physical Apple TV (Apple TV 4K 3rd gen / tvOS 26.6)

• dart analyze is clean for the package

• All four Pigeon channels round-trip on the simulator with no MissingPluginException.

• Real external launch confirmed two ways: on the simulator a registered app URL scheme launched a separate app (its AppDelegate logged the delivered URL); on a physical Apple TV 4K, launchUrl opened the App Store.

• In-app browser modes throw PlatformException(no_ui_available); supportsMode returns false for them.

Versioning & changelog

• version: set to 0.0.1 (new package)
• Matching ## 0.0.1 entry at the top of CHANGELOG.md
• Behaviour documented: in-app browser modes unsupported on tvOS
• Semver 0.x: initial 0.0.1

Checklist

• Only url_launcher_tvos files touched (+ the root README ports row, required by the R1 gate)
• No secrets, absolute local paths, or TODO/debug leftovers
• README.md documents the tvOS constraint (no in-app browser / no WebKit)
• Sibling note below

Notes for reviewers

• Generated Pigeon kept upstream-verbatim: messages.g.dart is byte-identical to 6.4.1; messages.g.swift differs only by the import gate (#if os(iOS) || os(tvOS)). The porter had wrapped UrlLauncherApiSetup.setUp in #if !os(tvOS) (it matched SFSafariViewController in a doc comment) — that would have unregistered every channel on tvOS; reverted.
• Native divergence is minimal + honest: URLLaunchSession (SafariServices) is #if !os(tvOS); openUrlInSafariViewController returns .noUI and closeSafariViewController is a no-op on tvOS. canLaunchUrl / launchUrl are untouched, and Launcher.swift / ViewPresenter.swift are byte-identical to upstream.
• Dart re-declares tvOS-honest supportsMode / supportsCloseForMode / platformDefault (this Dart runs only on tvOS, so no platform guards).
• Ships both a podspec and tvos/Package.swift (SPM), matching the pure-Swift method-channel plugins from feat(spm): add Package.swift to the Swift method-channel plugins #1.
• Version floor kept at the repo-standard flutter: >=3.13.0 — upstream 6.4.1 raised its floor to Flutter 3.38 / Dart 3.10 (in 6.4.0), but nothing in this tvOS slice needs it (it builds and dart analyzes clean on the lower floor), and it stays consistent with the sibling _tvos packages.
• Note: on tvOS canLaunchUrl can return true for a web URL even when nothing handles it, so callers should rely on the launchUrl return value (documented in the README). This PR also removes url_launcher from the README's "Evaluated but not provided" table, since it is now provided.