Add internal serializer for dot snapshot

[kamiazya]

HAS BRAKING CHANGE

What was a problem

When taking a DOT snapshot with the toBeValidDotAndMatchSnapshot API, it was treated as a string and the string was escaped, making the snapshot difficult to read.

How this PR fixes the problem

Created a serializer to avoid escaping strings.

Snapshot Diff

- exports[`public api toBeValidDotAndMatchSnapshot 1`] = `"digraph g { a -> b; }"`;
+ exports[`public api toBeValidDotAndMatchSnapshot 1`] = `digraph g { a -> b; }`;