Update the Figma plug-in to dynamic page loading

[GBKS]

I got an email from Figma that this migration needs to be done. Instead of loading all page at once when opening a file, now Figma only opens them as they are needed. That means plug-ins need to wait occasionally for content to be available to access it. Looks like the code just needs to be updated to be asynchronous. Does not sound too complicated. Here are the docs.

This can be done in conjunction with the update to the Bitcoin UI Kit plugin. See this issue.