Support code block formatting for string snapshots

[novemberborn]

As an overload in SnapshotAssertion:

ava/types/assertions.d.cts

Lines 320 to 332 in a6f42ea

	export type SnapshotAssertion = {
	/**
	* Assert that `expected` is [deeply equal](https://github.com/concordancejs/concordance#comparison-details) to a
	* previously recorded [snapshot](https://github.com/concordancejs/concordance#serialization-details), or if
	* necessary record a new snapshot.
	*
	* Returns `true` if the assertion passed and throws otherwise.
	*/
	(expected: any, message?: string): true;
	/** Skip this assertion. */
	skip(expected: any, message?: string): void;
	};

)

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 #2903 (reply in thread)

[novemberborn]

I'm doing some work on the library used for snapshotting, and it's causing double escaping when I snapshot the formatted values. This would avoid that problem.