Playground: Add ability to stop a running script #2541

When running some code in the playground, there is no way to stop the execution of a running program (a la Ctrl+C in the terminal). This can lead to some weird behaviour. For example, set an interval to log A to the console periodically and run the code. Changing the output to B causes A and B to both be output to the console.

Repro:

1. Simply run the following line of code in the playground, and run the code.

let interval = setInterval(() => { console.log('A') }, 1000);

2. Modify the code, and 'run' again:

let interval = setInterval(() => { console.log('B') }, 1000);

Expected behavior:
I expect only 'B' to be printed.
Actual behavior:
'B' and 'A' are being printed.