Peeks a Rune at the specified offset from the current beginning of the slice
without using the range Start or End, returns default if outside the Text.
Recognizes supplementary code points that cannot be covered by a single Char.
A positive offset value expects the high surrogate and a negative offset expects the low surrogate of the surrogate pair of a supplementary character at that position.
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public readonly Rune PeekRuneExtra(int offset)
The offset.