Automatically update v2 extensions to v3 when opened in Page Editor

[BLoe]

General cases:

• If it’s a @ + identifier, convert to var
• If there’s a {{ or {% convert to the template engine

A corner case we don’t need to support is blueprints with multiple extensions. (Because you can’t just update the runtime of one of the extensions).

UX:
Open something that is v2, and it has a message: “The Page Editor no longer supports bricks created for runtime v2. We’ve automatically tried converting this extension to use v3. Read more about this change and troubleshooting here (link to a notion page)”