refactor: extract modules from bot.zig

This commit creates a bunch of new modules that contain code and tests
for various concepts/implementations that used to exist inside
bot.zig.

Notable amongst these are:
- buffer.zig, which contain the circular buffer containing both
  backlog and outbox messages.
- parser.zig, which contain the parser used to parse commands from IRC
  messages.

src/root.zig

@@ -1,3 +1,6 @@
 //! By convention, root.zig is the root source file when making a library.
 
 pub const bot = @import("bot.zig");
+pub const buffer = @import("buffer.zig");
+pub const parser = @import("parser.zig");
+pub const commands = @import("commands.zig");