defined function is not registered in the core functions index

[iwb-rubentytgat]

Describe the bug
The defined function, while implemented and documented, does not appear to be registered as a core function.

To Reproduce

1. Given any ruleset referencing the core function defined
2. It will throw an error: Function defined not found. Called by rule foo.

Expected behavior
It should find the defined function.

Additional context
Origin of the issue is probably in this file: https://github.com/stoplightio/spectral/blob/v5.6.0/src/functions/index.ts

The defined function is not included in the CoreFunctions map, unlike all the other core functions.

[P0lip]

Fixed in 5.7.1