SplitString: fix platform-dependent newline split, retarget net10.0

[vladimir-pecanac-main]

Sample for Different Ways to Split a String in C#, which is being republished.

The defect this PR exists for

SplitStringIntoNewLines was called with new[] { Environment.NewLine } against a literal whose only line endings are \n. On Linux that separator is "\n", so the split returns five elements and the test passes. On Windows Environment.NewLine is "\r\n", the input holds no \r, no separator is ever found, and the split returns one element holding the whole string.

WhenSplittingAStringIntoNewLine_ThenReturnArrayOfSubstrings asserted Length == 5 against exactly that, so CI was green on a test that fails on a Windows developer's machine. Measured on this box (SDK 10.0.302) before the fix:

Environment.NewLine bytes: 13,10
OLD (Environment.NewLine) length = 1
NEW (["\r\n", "\n"]) length = 5

Both sides now split on ["\r\n", "\n"], so the assertion is true on every platform rather than on the one the runner happens to use. A second test covers genuinely mixed \r\n / \n input.

Also in here

• SplitStringUsingStringArrayWithoptions takes a StringSplitOptions parameter defaulting to None, so the existing test keeps its exact behaviour while Main can pass RemoveEmptyEntries on an input that actually has empty entries ("apple,,banana;;kiwi"). The old input had none, so the option it was demonstrating did nothing visible.
• SplitStringWithOptions plus four tests, covering all four StringSplitOptions results on the runtime's own documented input "a,,b, c, , d ,e". Run on net10.0:

None: [a] [] [b] [ c] [ ] [ d ] [e]
RemoveEmptyEntries: [a] [b] [ c] [ ] [ d ] [e]
TrimEntries: [a] [] [b] [c] [] [d] [e]
RemoveEmptyEntries, TrimEntries: [a] [b] [c] [d] [e]

Note that RemoveEmptyEntries on its own keeps the whitespace-only " ", which is the point of the pair.

• CountSplitRanges, one method demonstrating ReadOnlySpan<char>.Split into a stack-allocated Span<Range>, which allocates no substrings at all.
• net6.0 -> net10.0 on both projects. Packages: MSTest.TestAdapter / MSTest.TestFramework 2.2.8 -> 4.3.3, Microsoft.NET.Test.Sdk 17.1.0 -> 18.9.0, coverlet.collector 3.1.2 -> 10.0.1 (all read from the NuGet flat container today).

Verification

Windows, SDK 10.0.302:

Build succeeded.
    0 Warning(s)
    0 Error(s)

Passed!  - Failed: 0, Passed: 13, Skipped: 0, Total: 13, Duration: 53 ms - SplitStringTests.dll (net10.0)