Edit Visually browser extension – iteration 2

[adamziel]

🚧 A more involved description TBD 🚧

Large changes:

• Snappy loading
• WordPress, PHP, and everything needed to run the editor is now shipped with the extension.
• There's no more iframe and a modal. Instead, there's a separate editor window. Manifest v3 makes the iframe non-feasible.
• An external domain with a loopback service worker is used to work around Manifest v3 limitation
• Extension service worker is involved, which breaks Firefox compat (not sure about Safari)

cc @dmsnell