Support code block formatting for string snapshots

[novemberborn]

Fixes #3441.

Adds an overload for t.snapshot():

t.snapshot(expected, {formatAsCodeBlock}, message?)

Only accepted when expected is a string. The string is stored verbatim and rendered as a fenced code block in the resulting .snap.md file. Set formatAsCodeBlock to true for a generic code block, or a non-empty language identifier string for syntax highlighting:

test('generates code', t => {
	const code = generate();
	t.snapshot(code, {formatAsCodeBlock: 'js'});
});

This requires a new binary snapshot version, but we retain compatibility with v3 and only write v4 if the snapshot contains these formatted strings.