Fix#56, which had gone stale but is now reopened. The problem which it reported with Xlsx Writer was fixed long ago. However, Xls Writer has 2 problems regarding hyperlinks. First, some logic which should have been `if ... elseif ... else ...` was coded as `if ... if ... unconditional ...`, resulting in multiple writes and a corrupt worksheet (which Excel does fix correctly). Second, it treated a hyperlink url starting with `#` (pointer to a cell in the same spreadsheet) as external, when it should be treated as internal.
Also changed `Hyperlink::isInternal` to recognize a starting `#`, and to use `str_starts_with('sheet://')` rather than `str_contains`.
oleibman