Add net11 span white-space search APIs

[SimonCropp]

Adds the white-space searches added to ReadOnlySpan<char> in net11:

• ContainsAnyWhiteSpace()
• IndexOfAnyWhiteSpace()
• IndexOfAnyExceptWhiteSpace()
• LastIndexOfAnyWhiteSpace()
• LastIndexOfAnyExceptWhiteSpace()

API count 1047 to 1052.

Semantics

The white-space set was checked against net11 across the whole BMP and matches char.IsWhiteSpace on every code point, so the implementation is a straight scan using it rather than a hardcoded set. Empty spans return false and -1, matching net11.

The BCL versions are vectorised and these are not, but both are O(n) and the results are identical, so no //Note: was added.

Placement

In their own Polyfill_Memory_WhiteSpace.cs, following the existing split of Polyfill_Memory into Polyfill_Memory_StartsWith.cs, Polyfill_Memory_EndsWith.cs and so on. That also keeps this clear of #585, which touches the end of Polyfill_Memory.cs, so the two can merge in either order.

Note that MemoryExtensions.IsWhiteSpace(ReadOnlySpan<char>) is not included: it already ships in System.Memory, so it is available wherever FeatureMemory is set.

Verification

• Solution builds clean in Release. Consume builds clean in Debug across all 22 target frameworks.
• Tests green on net11.0 (1635), net462 (1612) and net8.0 (1632), plus PublicTests, EmbeddedTests, UnsafeTests, NoRefsTests and NoExtrasTests.
• The parameterised test covers space, tab, newline, carriage return, vertical tab, no-break space, em space and ideographic space, plus a letter and the zero width space, which is not white-space. The same assertions run against the BCL on net11 and against the polyfill on earlier target frameworks.