Break out source into multiple files

[klange]

Bim being a single C source file is a legacy holdover from ToaruOS's build system. Bim (~6850) is the largest application in ToaruOS at over twice the LoC of the compositor (~2750), and as such stands as an outlier for the single-file build system. While Bim's large size is useful for dogfooding, it has become unruly to manage and navigate, and some elements such as syntax highlighters and themes offer clear opportunities to break things up.