A new notepad member function is needed to support a plugin command new to N++ release 8.6.9: NPPM_SETUNTITLEDNAME

The new function should (ideally?) be callable as notepad.setUntitledName(newName, buffId) where newName is a string (of 63 characters or less and contains no illegal character) and buffId is a default arguments with value -1 (to mean active tab is being renamed).

Details on the N++ implementation:

• https://npp-user-manual.org/docs/plugin-communication/#2139nppm_setuntitledname
• notepad-plus-plus/notepad-plus-plus@b3daf0a
• Add plugin interface for renaming an unsaved tab notepad-plus-plus/notepad-plus-plus#8916
• Add plugin method to rename untitled tab notepad-plus-plus/notepad-plus-plus#15291