Fix and enable resizable splitting functionality

[NoahAndrews]

Key changes:

• The div that contains blocklyArea and output is now a horizontal flexbox container. This ensures that the two areas are always displayed side-by-side, and don't end up on separate lines
• We wait a few milliseconds before calling resizeBlocklyArea() for the first time. I wish I could tell you why this is necessary, but it fixes an issue where the split wouldn't render correctly until the window got resized. We had to do the same thing in the FTC Blocks environment.

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[ignatandrei]

Thanks! @all-contributors please add @NoahAndrews for code

[allcontributors[bot]]

@ignatandrei

I've put up a pull request to add @NoahAndrews! :tada: