perf: set capacity of List in MembersForcedLine

[TimothyMakkison]

Most result Lists are always just under 3 * members.Count, small change but simple 🤷

Benchmarks (timing is inaccurate should be the same)

Before

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	133.0 ms	2.56 ms	2.84 ms	3000.0000	1000.0000	34.58 MB
Default_CodeFormatter_Complex	271.5 ms	5.40 ms	7.92 ms	5000.0000	2000.0000	52.95 MB

After

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	132.4 ms	2.64 ms	3.87 ms	3000.0000	1000.0000	34.56 MB
Default_CodeFormatter_Complex	268.2 ms	4.92 ms	4.60 ms	5000.0000	2000.0000	52.78 MB