Reduce allocations in UPath.Combine

[GerardSmit]

This PR reduces the allocations in .NET 7 and higher

Benchmark

BenchmarkRunner.Run<Benchmark>();

[MemoryDiagnoser]
public class Benchmark
{
    [Benchmark] public UPath Combine2() => UPath.Combine("/a", "b");
    [Benchmark] public UPath Combine3() => UPath.Combine("/a", "b", "c");
    [Benchmark] public UPath Combine4() => UPath.Combine("/a", "b", "c", "d");
    [Benchmark] public UPath Combine5() => UPath.Combine("/a", "b", "c", "d", "e");

    [Benchmark] public UPath Combine2WithAbsolute() => UPath.Combine("/a", "/b");
    [Benchmark] public UPath Combine3WithAbsolute() => UPath.Combine("/a", "b", "/c");
    [Benchmark] public UPath Combine4WithAbsolute() => UPath.Combine("/a", "b", "c", "/d");
    [Benchmark] public UPath Combine5WithAbsolute() => UPath.Combine("/a", "b", "c", "d", "/e");
}

Before

Method	Mean	Error	StdDev	Gen0	Allocated
Combine2	37.39 ns	0.267 ns	0.250 ns	0.0019	32 B
Combine3	66.89 ns	0.415 ns	0.388 ns	0.0043	72 B
Combine4	106.16 ns	1.653 ns	1.546 ns	0.0062	104 B
Combine5	145.36 ns	1.328 ns	1.242 ns	0.0134	224 B
Combine2WithAbsolute	15.62 ns	0.072 ns	0.067 ns	-	-
Combine3WithAbsolute	46.70 ns	0.804 ns	0.752 ns	0.0019	32 B
Combine4WithAbsolute	58.17 ns	0.912 ns	0.853 ns	0.0019	32 B
Combine5WithAbsolute	122.32 ns	1.402 ns	1.311 ns	0.0105	176 B

After

Method	Mean	Error	StdDev	Gen0	Allocated
Combine2	34.48 ns	0.292 ns	0.259 ns	0.0019	32 B
Combine3	47.70 ns	0.201 ns	0.178 ns	0.0024	40 B
Combine4	64.36 ns	0.485 ns	0.430 ns	0.0024	40 B
Combine5	146.00 ns	1.028 ns	0.802 ns	0.0134	224 B
Combine2WithAbsolute	15.94 ns	0.058 ns	0.051 ns	-	-
Combine3WithAbsolute	27.21 ns	0.088 ns	0.078 ns	-	-
Combine4WithAbsolute	37.27 ns	0.060 ns	0.050 ns	-	-
Combine5WithAbsolute	125.59 ns	0.470 ns	0.392 ns	0.0105	176 B