Describe "safe mode" for JSON-LD processing.

[BigBlueHat]

JSON-LD Safe Mode has been part of the jsonld.js implementation since 2022:
https://github.com/digitalbazaar/jsonld.js?tab=readme-ov-file#safe-mode

There is an interest from other communities for this to be a core part of the JSON-LD API as the current default of ignoring unmapped terms (for example) is seen as a risk when attempting to enforce explicit JSON shapes.

@dlongley and @davidlehn can provide more details.