reduce number of Unicode private-use characters

This narrows the range of Unicode codepoints fish reserves for its own
use from U+E000 thru U+F8FE (6399 codepoints) to U+F600 thru U+F73F (320
codepoints). This is still not ideal since fish shouldn't be using any
Unicode private-use codepoints but it's a step in the right direction.

This partially addresses issue #2684.

src/tokenizer.cpp

@@ -632,8 +632,8 @@ void tokenizer_t::tok_next()
             /*fwprintf( stderr, L"End of string\n" );*/
             this->has_next = false;
             break;
-        case 13: // carriage return
-        case L'\n':
+        case L'\r': // carriage-return
+        case L'\n': // newline
         case L';':
             this->last_type = TOK_END;
             this->buff++;