Add support for inline image in markdown editor by drag and drop of an image file

[HiroyasuNishiyama]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

This PR adds inline figure support to the markdown editor of Node-RED editor used for describing node's as documents discussed at this forum thread.

• inserting an inline image as data URL () by drag & drop of an image file.

This PR is one of two features included in the previous PR.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage: 68.241% (-0.005%) from 68.246% when pulling e7540de on node-red-hitachi:add-markdown-image-drag-and-drop into 928131c on node-red:dev.

[knolleary]

Looks good.

Do you think we should put some limits in place based on file extension and/or size?

If a user starts dropping in lots of 1+Mb images then they could quickly hit API payload size limits. Also, the file type - we should probably limit to png/jpeg/gif.

[HiroyasuNishiyama]

Do you think we should put some limits in place based on file extension and/or size?

If a user starts dropping in lots of 1+Mb images then they could quickly hit API payload size limits.

Considering that multiple images can be used in a flow, I think it can be difficult to determine the maximum size of droppable image file.

Also, the file type - we should probably limit to png/jpeg/gif.

According to this page, apng, avif, svg, and webp are other candidates.
I will update the code to prevent uploading unexpected file type.

[knolleary]

Had to remove package-lock.json that had accidentally slipped in - but otherwise looks great