Fix Skulpt External Files

[sra405]

Fix:

• add external files to document body so it renders outside the shadow DOM

Why:

• Skulpt picks up files by adding them to the DOM (see import and file read skulpt/skulpt#854 (comment))
• this doesn't play nice with the shadow DOM and Skulpt doesn't recoginse files like .txt and .csv
• createPortal enables us to pick a common host element (i.e. body) and attach the required div's there

[github-actions]

• Github Run 10269632675
• Test coverage: 78%

[conorriches]

s p l e n d i d