add StringScanner#rest to the list of methods that look ahead/behind

Author: jneen

src/string_scanner.cr

@@ -45,6 +45,7 @@
 # * `#peek`
 # * `#check`
 # * `#check_until`
+# * `#rest`
 # * `#current_char`, `#current_char?`
 # * `#previous_char`, `#previous_char?`
 # * `#current_byte`, `#current_byte?`