perf: use `FileSystemEnumerator` #235

Job	Run time
Run Tests	1m 12s
No Warnings	57s
Check Formatting	15s
Test CSharpier.MSBuild (ubuntu-latest)	1m 33s
Check TODOs	11s
Builds Playground	57s
Test CSharpier.MSBuild (windows-latest)	2m 8s
	7m 13s

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