Unicode in mapgen

[jbytheway]

Summary

SUMMARY: Infrastructure "Allow Unicode keys in mapgen rows"

Purpose of change

Some mapgen palettes (such as the one for houses) are running out of useful ASCII characters to use as keys.

Describe the solution

Allow arbitrary Unicode characters as keys, provided they consume only a single column (so e.g. no Chinese characters, nor most emojis).

Combining characters are handled, so arbitrary accents and even Z̴̨̧̞̦̱̭̼̹̱̠̱͍͙̥̃̽͋̅͝ā̴̛̛̬̞͈̞͇̀̏̐́̎͆́̚ͅl̵̖͔̥̑̓͛͑̇̃͋̑̈́̀͆̅̓̕͝g̶̨̗͋̆̆̈́̀̋̌͗͝o̴̥̥̬̥̱͆̀̍͐͌͑̍͘̕͠ ̴̺̫̹̂̏̇͒̀͂̈̌͌̉̓̔͆̽͘͜ẗ̷̛̲͈̱͚̣͔̥͍̗͔͖́̅͊̀͛̎͒͠͠e̷̙̣͇̗͖͈͔̲̤̞̥̱̤̜͚͆̀̈́̎̈́̈́̒͂͠͝x̴̢̢̨̨̫̬͇̱͖̯̠͙̬̩͚̓̑̆̿̌̌́͘t̷̡̮̓̓͗͛͋͒̆̑̀̌̌̒ should work. No normalization is performed.

Add a single example use of this feature in house_w_palette by using a box-drawing character that looks somewhat like a bookcase (▤) to represent bookcase.

Document the new feature.

Describe alternatives you've considered

We could try to allow double-width characters. We'd probably want to enforce that the entire map used a single width of character consistently. But this seems like enough generalization for now.

Could try to normalize the strings so that different representations of the same character are equivalent, but we don't have the Unicode data required to do that, and I didn't want to add a dependency on libicu just for this.

Testing

Used the new feature in house_w_palette as mentioned above. Spawned some of those houses in-game to verify that bookcases and books still appear.

Additional context

Requested by @I-am-Erk.