feat: set stage size initialState to small

This change means that Scratch starts with the stage (where users can run their
code) in the "small" mode. They can then toggle to "large" or "fullscreen". The
reason for this change is to make the scratch interface we embed in Experience
CS more suitable for narrower screens, where we still need to leave room for the
instruction panel next to Scratch.

I considered making a more extensive modification to gui.jsx where it calls
`resolveStageSize` to take into account the size of the containing element,
rather than the size of the screen/browser window (I think this code will have
been written with the assumption that Scratch is taking over the whole
viewport).

In the end I decided against this - it'd be a more invasive change and one that
would probably require quite a bit more testing and longer-term maintainance in
case of upstream changes. Given that the user has the option to toggle to a
larger stage themselves, I think for now this very small change to the default
is preferable.

Co-authored-by: Chris Lowis <chris.lowis@gofreerange.com>

Author: sebjacobs

packages/scratch-gui/src/reducers/stage-size.js

@@ -3,7 +3,7 @@ import {STAGE_DISPLAY_SIZES} from '../lib/layout-constants.js';
 const SET_STAGE_SIZE = 'scratch-gui/StageSize/SET_STAGE_SIZE';
 
 const initialState = {
-    stageSize: STAGE_DISPLAY_SIZES.large
+    stageSize: STAGE_DISPLAY_SIZES.small
 };
 
 const reducer = function (state, action) {