Add CommunityToolkit.Maui.SourceGenerator.Benchmarks, Reduce Memory Allocations on [BindableProperty] Source Generator

[TheCodeTraveler]

Description of Change

This PR adds CommunityToolkit.Maui.SourceGenerator.Benchmarks.csproj and updates the GitHub Action benchmarks.yml.

This PR also optimizes the [BindableProperty] Source Generator to reduce memory allocations.

PR Checklist

• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

This PR updates benchmarks.yml to only run on Windows. This was done for two reasons:

1. We only publish the results from the Windows benchamarks
2. The Source Generator Benchmarks do not run successfully on macOS

[ne0rrmatrix]

This looks good to go.