perf: set capacity of `List` in `MembersForcedLine` #317

Job	Run time
Check Formatting	12s
Run Tests	1m 10s
Test CSharpier.MSBuild (windows-latest)	1m 44s
Builds Playground	57s
Check TODOs	7s
No Warnings	54s
Test CSharpier.MSBuild (ubuntu-latest)	1m 24s
	6m 28s

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