Add microphone input

## What problem does this solve or what need does it fill?

Microphone inputs can be useful in various game contexts, either as a gameplay element or as the foundation of voice chat.

## What solution would you like?

Add first-party Bevy support, within bevy_audio but behind a feature flag or in a separate crate.

## What alternative(s) have you considered?

Implement this by plumbing together an existing crate on your own. While possible, this is nice to handle with a Bevy-specific integration, to ensure that we have high-quality cross-platform support and a simple ECS-native abstraction layer.

Start by examining the existing bevy_audio support and the options for placing microphone input behind a feature flag or in a separate crate. Compare existing Rust audio-input crates for cross-platform coverage and determine the ECS-native API and integration scope. Done means first-party microphone support is implemented with a clear crate or feature boundary and documented platform behavior.

Written by the indexing model from the issue text.