Issue 23943 - allow negative indices in substring to remove from the end of the string

[Famlam]

https://josm.openstreetmap.de/ticket/23943

Summarized, see ticket for details:

• Current behavior: StringIndexOutOfBoundsException
• Current methods to remove a fixed number of characters from the end of the string: ugly workarounds are necessary to obtain the desired behavior (unless the length of the string is already known beforehand, in which a hardcoded index can be used)
• Proposed method: allow negative end indices to remove characters from the end of the string