perf: optimise `IgnoreFile` using `ValueTask` and `Span.Replace` #215

Job	Run time
Run Tests	1m 13s
No Warnings	57s
Check Formatting	17s
Builds Playground	1m 5s
Check TODOs	8s
Test CSharpier.MSBuild (windows-latest)	2m 31s
Test CSharpier.MSBuild (ubuntu-latest)	1m 42s
	7m 53s

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