Allow customMessage to be a function that returns message

[Coridyn]

What

Feature: allow the customMessage parameter to be passed a function as well as a string

// Original (and still supported)
expect(value, `custom message string`).toBe(actual);

// New: allow a function that returns the message
expect(value, () => (`custom message from function`)).toBe(actual);

Why

This allows the generation of customMessage to be lazy - i.e. the message will only be generated if the expectation failed.

If the message generation is expensive to do then the cost is only incurred when the test failed.

This also makes the customMessage API more consistent with Jest v27+ that requires the matcher.message property to be a function.

Housekeeping

• Unit tests
• Documentation is up to date
• No additional lint warnings