Support improperly terminated sequences in escape_parse

For example, for "\e\e[31mHello", continue parsing instead of exiting
escape mode at the second \e

src/flanterm.c

···
     }
 
     switch (c) {
+        // Got ESC in the middle of an escape sequence, start a new one
+        case 0x1B:
+            return;
         case 'F':
             x = 0;
             // FALLTHRU