Added Benchmark project comparing performance of 3.1.0 vs 3.2.0 (encode and decode)

[loudenvier]

This PR introduces a new Sqids.Benchmarks project to the solution, providing initial performance benchmarks for both encoding and decoding operations.

Key Details

• Includes a custom BenchmarkDotNet configuration.
• Adds conditional project variables to reference both old and new package versions, enabling fair performance comparisons across releases.
• Benchmarks encode/decode of int.MaxValue, using either int or long depending on availability of long support:
  • Fully supported in v3.2.0.
  • Available only in the generic version of v3.1.0.
• Ensures consistent input across versions for accurate comparison.

Results

• Performance impact is negligible in both speed and memory usage.

Method	Runtime	Arguments	Mean	Error	StdDev	Gen0	Allocated
SqidEncode	.NET 7.0	/p:BenchmarksPackageVersion=3.1.0	13,515.13 ns	137.628 ns	122.004 ns	0.6866	11680 B
SqidDecode	.NET 7.0	/p:BenchmarksPackageVersion=3.1.0	94.89 ns	0.334 ns	0.313 ns	0.0052	88 B
SqidEncode	.NET 7.0	/p:BenchmarksPackageVersion=3.2.0	13,846.03 ns	93.602 ns	73.079 ns	0.6866	11712 B
SqidDecode	.NET 7.0	/p:BenchmarksPackageVersion=3.2.0	97.26 ns	1.931 ns	1.896 ns	0.0052	88 B
SqidEncode	.NET 8.0	/p:BenchmarksPackageVersion=3.1.0	8,690.65 ns	40.099 ns	37.508 ns	0.6866	11680 B
SqidDecode	.NET 8.0	/p:BenchmarksPackageVersion=3.1.0	81.46 ns	0.659 ns	0.584 ns	0.0052	88 B
SqidEncode	.NET 8.0	/p:BenchmarksPackageVersion=3.2.0	8,729.78 ns	73.010 ns	68.293 ns	0.6866	11680 B
SqidDecode	.NET 8.0	/p:BenchmarksPackageVersion=3.2.0	80.47 ns	0.084 ns	0.075 ns	0.0052	88 B
SqidEncode	.NET Framework 4.8	/p:BenchmarksPackageVersion=3.1.0	50,760.07 ns	243.170 ns	227.461 ns	1.8311	11714 B
SqidDecode	.NET Framework 4.8	/p:BenchmarksPackageVersion=3.1.0	459.91 ns	0.476 ns	0.398 ns	0.0124	80 B
SqidEncode	.NET Framework 4.8	/p:BenchmarksPackageVersion=3.2.0	51,174.86 ns	172.228 ns	161.102 ns	1.8311	11899 B
SqidDecode	.NET Framework 4.8	/p:BenchmarksPackageVersion=3.2.0	533.62 ns	2.948 ns	2.758 ns	0.0439	281 B

Notes

• During repo sync after the previous PR merge, ~12 extra commits appeared, but they contain no actual changes. This should not negatively affect the repository.
• No modifications were made to the core codebase.
• No new NuGet package is required.