docs(pos): expose app background event methods

[vctrchu]

What

Adds a docs-only Docs_AppBackgroundEventMethods interface that picks addEventListener and removeEventListener from BackgroundShopifyGlobal, then regenerates the POS docs data.

Why

These methods live on the shopify global for app background extensions. They aren't a named target API, so the docs need a named type to render the methods without changing DataTargetApi or the target support mapping.

Testing

• yarn type-check
• yarn lint
• git diff --check