NPC/elite generation rework

[tukkek]

For World and Dungeon Random Encounters, instead of having pregenerated NPC and elite units, have instead a 10% chance of generating an elite/NPC encounter. Do this dynamically by generating an EncounterIndex with all Monsters in the encounter table, in a range like [el/2,el-2], then select a random sample of units from the generated encounter and iteratively upgrade the weakest until the target EL is reached.

If a special encounter can't be generated, just fall-through with the normal encounter selection.

The problem with this approach is that it will exclude elites and NPCs from other content - this isn't necessarily terrible as Haunts and Sieges are more intended to be larger-scale battles where NPCs would be interesting but not necessary and possibly hard to keep track of.

A future issue can be opened to investigate future possibilities for other cases (Haunt, Incursions, garrisons)... - like pre-generating level 5/10/15/20 NPCs/elites for each favored Kit rather than the current approach. This would result in 5x the entries for each creature, pet Kit so an alternative approach would be to target a number of NPCs per Kit (2, for example) and generate said NPCs from there (maybe one at low level, similarly to Arena and one from any available Monster).

If an universal approach with the 10%-chance strategy can be easily done in Encounter Generator directly, even better.