perf: add Concat(IEnumerable) overload and remove ToArray

[TimothyMakkison]

• Add Concat(IEnumerable) overload to remove ToArray call. This is beneficial because most uses only have one item so we don't have to create an array.
  • Synergises nicely with perf: add new Concat types for children lengths #1524
• Prevent repeat calls to syntaxToken.LeadingTrivia
• Use ValueListBuilder inside Doc.Join

Benchmark (Timing is inaccurate)

Before

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	131.9 ms	3.30 ms	8.86 ms	128.0 ms	3000.0000	1000.0000	34.57 MB
Default_CodeFormatter_Complex	261.3 ms	5.16 ms	4.03 ms	261.3 ms	5000.0000	2000.0000	53.03 MB

After

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	139.0 ms	2.96 ms	8.59 ms	3000.0000	1000.0000	34.35 MB
Default_CodeFormatter_Complex	282.8 ms	5.59 ms	13.39 ms	5000.0000	2000.0000	52.53 MB