PERF: Optimize StringExtensions.cs

[perf-cat]

Hi, I've tried to make some performance optimizations to this repo. Following are the benchmark results before and after the changes:

Benchmarks before changes:

Method	Mean	Error	StdDev	Gen 0	Allocated
OnlyNumbers	161.3 ns	2.40 ns	2.13 ns	0.0072	136 B
NetDevPackOnlyNumber	149.5 ns	3.09 ns	2.89 ns	0.0103	192 B

Benchmarks after changes:

Method	Mean	Error	StdDev	Gen 0	Allocated
OnlyNumbers	165.4 ns	3.31 ns	4.54 ns	0.0072	136 B
NetDevPackOnlyNumber	126.9 ns	2.68 ns	4.61 ns	0.0076	144 B

As you can see the durations and allocations have gone down. I've also made sure that all the unit tests pass following these changes as well. Could you please confirm whether or not you think my changes are valid?

Thank you!