Make ReadOnlySpan<char>.StartsWith/EndsWith(string) ordinal

[SimonCropp]

Bug

Below netcoreapp3.0 the polyfills were declared as

EndsWith(this ReadOnlySpan<char> target, string other, StringComparison comparison = StringComparison.CurrentCulture)
StartsWith(this ReadOnlySpan<char> target, string other, StringComparison comparison = StringComparison.CurrentCulture)

so span.EndsWith("c") did a current-culture comparison. On newer frameworks the same call binds to MemoryExtensions.EndsWith<T>(ReadOnlySpan<T>, ReadOnlySpan<T>), which is ordinal. For example "abc­".AsSpan().EndsWith("c") returned true on net48 and false on net10.0 (the soft hyphen is ignored by culture-sensitive comparison).

Fix

Each method is split into two overloads: one without a comparison that does the ordinal sequence comparison, and one with a required StringComparison. The API count goes up by two as a result.

Test

ReadOnlySpan_StartsWith_String_IsOrdinal and ReadOnlySpan_EndsWith_String_IsOrdinal in PolyfillTests_Memory.cs check that the no-comparison call does not ignore a soft hyphen, that the explicit InvariantCulture call does, and that an exact ordinal match still succeeds. Both failed on net48 before the fix and pass on net48 (polyfill) and net10.0 (BCL) after it.