Feature request: Support negative from-the-end indexes in `INCBIN`

We currently support `INCBIN "path.bin"[, start[, len]]` to include *len* bytes (or "the rest" of the bytes if it's not specified) starting at index *start* (or the beginning of the file, index 0, if it's not specified). However, we do *not* support negative from-the-end indexes. This use case is rare but not unheard of, and we generally do support negative indexes in other features (string functions, angle-bracketed macro arguments, etc).

Start by locating the INCBIN implementation and its existing tests, then read how start and len are parsed and applied. Check the handling of negative indexes in the other features mentioned by the issue. Done means INCBIN accepts negative from-the-end indexes while preserving current behavior for omitted and non-negative values, with coverage for the new cases.

Written by the indexing model from the issue text.