Drag and drop fixes

[microbit-matt-hillsdon]

• Default to appending to existing blocks but allow control over the indent level where it can't cause invalid indentation.
• Fix handling of whitespace in "while smooshing"

Closes #856.
Closes #812.
Closes #800.
Closes #539.

Concerns:

• As we don't detect intent level this weds us further to 4 space indents. Easily generalised if we did autodetect or have a preference in future. I think if you do either you need to do both though. Code structure highlighting is enabled by default and looks very poor if you don't follow the convention so I think this is OK. Likewise the autoindent and tab behaviours.
• Build change to allow iterating over generators. Needs a quick check in supported browsers but really should be fine.

[github-actions]

Preview build will be at
https://review-python-editor-v3.microbit.org/alt-dnd-fixes/

[microbit-matt-hillsdon]

Build change to allow iterating over generators. Needs a quick check in supported browsers but really should be fine.

This looks good. I don't think it's really changed anything other than the input to babel.