feat: Add Swift Package Manager support for iOS and macOS

[vasu-nageshri]

Description

Migrate plugin sources to SPM-compatible directory structure and add Package.swift for iOS and macOS with FlutterFramework dependency.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #492

[bbbshah]

Thanks for tackling this — the overall approach is right: FlutterFramework package dependency, correct Sources/audio_waveforms/ layout, and the hyphenated audio-waveforms product name. A few things should be addressed before merge:

1. macOS: duplicate class + infinite recursion (build/runtime breaker).
On iOS you correctly deleted the Obj-C shim and renamed the Swift class to AudioWaveformsPlugin. On macOS the Obj-C shim was kept and the Swift class was renamed to the same name, so there are now two AudioWaveformsPlugin classes, and the shim registers itself:

+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar> *)registrar {
    [AudioWaveformsPlugin registerWithRegistrar:registrar]; // recurses into the Obj-C class
}

Please mirror the iOS change on macOS: delete include/audio_waveforms/AudioWaveformsPlugin.h and AudioWaveformsPlugin.m and rely on the Swift AudioWaveformsPlugin (pubspec already points macos.pluginClass at it). The publicHeadersPath/headerSearchPath entries in the macOS Package.swift can then be dropped.

2. Flutter SDK constraint is too low. The .package(name: "FlutterFramework", path: "../FlutterFramework") dependency is only generated by Flutter 3.44+. pubspec.yaml should be bumped to align with the official guidance (flutter: ">=3.41.0", sdk: ^3.11.0) rather than >=3.22.0, otherwise SwiftPM resolution fails on older SDKs.

3. iOS deployment target mismatch. Package.swift declares .iOS("13.0") but the podspec is set to 9.0. These should match — please set the podspec to :ios, '13.0'.