Implement 'printf' utility

This is by no means complete, but covers most of the POSIX spec:
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/printf.html

- Escape sequences (currently the shell handles these wrong, see
  HeyPuter#53 )
- %d %i %o %u %x and %X integer formatters
- %e %E %f %F %g and %G float formatters
- %c and %S string formatters

Still to do:
- %a and %A hexadecimal float formatters
- %b for formatting a string including escape sequences
- Anything beyond the spec that's common and users might expect.

src/puter-shell/coreutils/__exports__.js

@@ -39,6 +39,7 @@ import module_man from './man.js'
 import module_mkdir from './mkdir.js'
 import module_mv from './mv.js'
 import module_neofetch from './neofetch.js'
+import module_printf from './printf.js'
 import module_printhist from './printhist.js'
 import module_pwd from './pwd.js'
 import module_rm from './rm.js'
@@ -77,6 +78,7 @@ export default {
     "mkdir": module_mkdir,
     "mv": module_mv,
     "neofetch": module_neofetch,
+    "printf": module_printf,
     "printhist": module_printhist,
     "pwd": module_pwd,
     "rm": module_rm,