Add the beginnings of a generic prompt system, with exclusive support
atm for string prompts, and test it out by asking the character for
their name at startup
Add support for converting generated levels to walls, and merge one into
the entity map at the beginning of the game.
There's nothing here that guarantees the character ends up *inside* the
level though (they almost always don't) so that'll have to be slotted
into the level generation process.
Add a "previous message" command, triggered via ctrl+p.
I attempted here to get the message area to still take up a row of space
post-hiding the message, but failed - should revisit that at some point
