Feature request: New script button

## Feature
Add a 'new' button that create's a new blank script in the editor

## Issue
It isn't obvious how to start a new script in the editor. I can select all my code and delete, but that will keep the program name in the window. I can also refresh the page which offers a warning before resetting the editor to it's default status, but this is a less common UX eg forms often maintain data on refresh and the MakeCode editor also does, so we should look to implement a similar feature.

## Alternatives
We can refresh the editor to create a new script or delete code, so we have options, maybe we call this out more in help.html?