FileAttachments: Stdlib Implementation

[jashkenas]

... for use in UI, API (for downloads), and embeds.

• UI: resolve() function posts messages up to the parent frame to get signed URLs for private files.
• API: resolve() function is compiled with relative paths to downloaded files in the folder hierarchy.
• Embeds: resolve() function is compiled into the ES Modules, with absolute URLs of published files.

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Update

• Embeds: resolve() function is compiled into the generated ES Modules, with a map from name to absolute public (CloudFront), or private (currently CloudFront, future API) URLs.
• Downloads: resolve() function is compiled into the generated ES Modules, with a map from name to relative path on disk.
• Imports: Identical to Embeds, compiled with absolute URLs.
• UI: Passes in a resolve() function to this implementation that is capable of asking the parent frame (which may ask the server) for the URL for a given file.

[mbostock]

We should add a section to the README before this lands.