perf: add ValueStringBuilder

[TimothyMakkison]

Add ValueStringBuilder to replace the StringBuilder allocation in SeparatedSyntaxList.Print saves 0.5MB

• Uses MemoryExtensions.Trim to avoid allocating an extra string
  • .NET Standard 2.0 doesn't have this method. I opted to add a simple allocating version as I assume .NET Standard performance doesn't matter as much.
  • I can copy over the relevant performance code for .NET Standard if you'd prefer
• Uses SkipLocalsInit to get around the minor zeroing cost during prolog, this isn't essential

Do you think this PRs complexity is worth adding it's not a massive saving.

[belav]

If the only benefit is 0.5MB I don't think it would be worth the added complexity.