Array.AppendAll can append a selected source range

Array.AppendAll currently appends every element from a whole source array. Add overloads that select a range of the source array so callers can append a contiguous suffix or sub-range without first copying it into a temporary array:

• AppendAll(items As Array(Of T), startIndex As Integer) appends from startIndex to the end of items.
• AppendAll(items As Array(Of T), startIndex As Integer, count As Integer) appends exactly count elements from [startIndex, startIndex + count).

startIndex may equal the source count and count may be zero (no-op). Negative or out-of-the-source-bounds ranges and ranges that extend beyond the source raise a runtime error. A partial self-append appends a snapshot of the selected source range. The methods mutate the receiver, return Void, and preserve the source element order and shallow-copy semantics of the existing whole-array overload.