[iOS][globalization] Implementing locale-sensitive `IsPrefix` with correct `matchLength` output parameter on iOS

[matouskozak]

Based on the current documentation

Apple Native API does not expose locale-sensitive endsWith/startsWith function. As a workaround, both strings get normalized and weightless characters are removed. Resulting strings are cut to the same length and comparison is performed. As we are normalizing strings to be able to cut them, we cannot calculate the match length on the original strings. Methods that calculate this information throw PlatformNotSupported exception.

runtime/src/libraries/System.Private.CoreLib/src/System/Globalization/CompareInfo.cs

Lines 615 to 621 in b61c8fc

	// Linguistic comparison requested and we don't need to special-case any args.
	#if TARGET_MACCATALYST || TARGET_IOS || TARGET_TVOS
	if (GlobalizationMode.Hybrid)
	{
	throw new PlatformNotSupportedException(SR.PlatformNotSupported_HybridGlobalizationWithMatchLength);
	}
	#endif

We should remove the exception throw for IsPrefixx/IsSuffix APIs and instead document the differences in returned matchLength or improve the implementation to give correct results.

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