docs: add pasteImage to demo and document the event

[SethFalco]

• The demo currently does nothing when we paste images. This inserts the image inline, so this can be demonstrated.
• Addressed typos in Demo.html and README
• Changed the link to Squire's source-code to the up-to-date one, so no redirect is required.
• By running npm i, it made package-lock.json up-to-date with changes from package.json, i.e. version.
• Ran npm run build, which changed files despite no changes being made to src in this PR. :thinking:

Related

• Closes https://github.com/fastmail/Squire/issues/425
• Related to https://github.com/tutao/tutanota/pull/5275

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Please consider reviewing/merging the following PR first:

• https://github.com/fastmail/Squire/pull/427

[neilj]

Thanks, much appreciated!