perf: replace StringBuilder with ValueStringBuilder

[TimothyMakkison]

Replaces StringBuilder with ValueStringBuilder

• Might be worth checking if key[1..].ContainsUpper() and early returning
• ValueStringBuilder uses the ArrayPool.Rent contructor, it could be worth doing conditional creation either using this, or using stackalloc char[]

For example:

var minCapacity = key.Length + SeparatorCapacityHeadroom;
var builder = minCapacity < 256 ? new ValueStringBuilder(stackalloc char[256])
    : new ValueStringBuilder(minCapacity);