Add a Lines(this string) extensions method

[snowfrogdev]

This method is inspired by rust's String.lines() method.

I've decided to make this method return a IEnumerable<ReadOnlyMemory<char>> instead of an IEnumerable<string> because there is a non-negligeable performance gain in doing so.

I've compared three different ways of splitting a string on new lines, here are the benchmarks:

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.22000
12th Gen Intel Core i9-12900K, 1 CPU, 24 logical and 16 physical cores
.NET Core SDK=6.0.303
 [Host]     : .NET Core 6.0.8 (CoreCLR 6.0.822.36306, CoreFX 6.0.822.36306), X64 RyuJIT
 DefaultJob : .NET Core 6.0.8 (CoreCLR 6.0.822.36306, CoreFX 6.0.822.36306), X64 RyuJIT

Method	N	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
LinesSplit	1	17.925 ms	0.3575 ms	0.9480 ms	1.00	0.00	687.5000	406.2500	125.0000	9556.79 KB
LinesSubstring	1	13.590 ms	0.2694 ms	0.5254 ms	0.74	0.04	703.1250	421.8750	140.6250	9081.89 KB
LinesMemory	1	13.551 ms	0.1991 ms	0.1554 ms	0.74	0.03	703.1250	421.8750	140.6250	9081.97 KB
LinesSplit	5	15.722 ms	0.3055 ms	0.3637 ms	1.00	0.00	687.5000	406.2500	140.6250	9174.87 KB
LinesSubstring	5	5.350 ms	0.1062 ms	0.1304 ms	0.34	0.01	562.5000	281.2500	-	8699.88 KB
LinesMemory	5	3.831 ms	0.0466 ms	0.0413 ms	0.24	0.01	117.1875	58.5938	-	1825.64 KB
LinesSplit	10	15.499 ms	0.3018 ms	0.5207 ms	1.00	0.00	687.5000	421.8750	140.6250	9119 KB
LinesSubstring	10	4.746 ms	0.0823 ms	0.0808 ms	0.30	0.01	562.5000	281.2500	-	8643.97 KB
LinesMemory	10	3.482 ms	0.0405 ms	0.0379 ms	0.22	0.01	58.5938	27.3438	-	904.15 KB
LinesSplit	15	14.510 ms	0.2732 ms	0.4564 ms	1.00	0.00	671.8750	406.2500	140.6250	9095.12 KB
LinesSubstring	15	4.466 ms	0.0856 ms	0.0800 ms	0.31	0.01	562.5000	281.2500	-	8620.04 KB
LinesMemory	15	3.390 ms	0.0424 ms	0.0354 ms	0.23	0.01	39.0625	15.6250	-	601.28 KB

[ardalis]

For benchmarks I'd think having N vary by a factor of 10 (like you did in your recent video) would be more useful than values of 1/5/10/15, just FYI

[ardalis]

Consider ensuring benchmark methods return identical results

[ardalis]

I like that you include a code example in the XML comments!