console: Use readline callback-based API
Changes gjs_console_readline() to use the readline callback-based API and process events from the default main context in between character inputs. This makes the console REPL non-blocking; for example, you can enter
setTimeout(() => console.log('now'), 1000)
and you'll get the log message after 1 second. Previously you would get it only after the REPL exited.
Closes: #76 (closed)