ref(input): move mouse.zig and key.zig into public input.zig namespace

src/event.zig

@@ -1,11 +1,12 @@
 //! Events which are defined by the library. They might be extended by user
 //! events. See `App` for more details about user defined events.
 const std = @import("std");
+const input = @import("input.zig");
 const terminal = @import("terminal.zig");
 
-const Mouse = @import("mouse.zig").Mouse;
+const Key = input.Key;
+const Mouse = input.Mouse;
 const Size = @import("size.zig").Size;
-const Key = @import("key.zig").Key;
 
 /// System events available to every `zterm.App`
 pub const SystemEvent = union(enum) {