Incorrect return type for `extract` method

[ben-tilden]

Description

The return type for extract is derived incorrectly which leads to typing inconsistencies.

A fix is already open for review.

In the ExtractedValue type, V['value'] is not distributed across conditionals as expected, leading to incorrect typing for extract's return type.

Current Behavior

type Value = ExtractedValue<ExtractValue, ExtractMap> is string | string[] | undefined.

Expected Behavior

type Value = ExtractedValue<ExtractValue, ExtractMap> should be unknown, based on ExtractDescriptorFn's unknown return type.