specenum generator: auto-generate generic (user flag) values
Just noticed that we can't even showcase this with the current def (terrain enums) because the terrain type user flags are followed by an extra TER_USER_LAST (which seems to be used much like a TER_COUNT would?).
Attached patch has textual dependency on #48789.
Follow-up idea: #48792
Add an option to generate_enums.py to generate a specified number of enum values with no names (e.g. for ruleset-defined user flags) using a specified prefix (e.g. USER_FLAG_)