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?

Start by tracing how the editor currently resets or creates scripts, using the editor entry point described in the issue. Review help.html as the suggested documentation alternative, then verify that the new button creates a blank script with an appropriate program name without requiring a page refresh.

Written by the indexing model from the issue text.