Support code block formatting for string snapshots

As an overload in SnapshotAssertion: https://github.com/avajs/ava/blob/a6f42ea472cfbe7047a0fb89ab0cd728b4e04ae1/types/assertions.d.cts#L320-L332)

snapshot(expected: string, options?: { formatAsCodeBlock: true | string }, message?: string)

This options object should only be accepted for string values, and must only contain formatAsCodeBlock. string values would be the syntax, and true would be the same as an empty string (but less weird).

When formatting, backticks would need to be escaped properly.

Originally posted by @novemberborn in https://github.com/avajs/ava/discussions/2903#discussioncomment-13017478