Add diffToolBuilder for building complex diff command messages

[js]

This PR adds SnapshotTesting.diffToolBuilder closure for building complex diff tool command messages or clickable urls in Xcode. It takes the existing and the failure snapshot as arguments, and thus complex commands which are not just prefixed with the existing diffTool can be created.

For instance, to open Differati from a clickable link in Xcode the following diffToolBuilder closure can be provided, which URL encodes the paths (in case they contain spaces etc) and builds an url openable by the app:

SnapshotTesting.diffToolBuilder = { existing, failed in
    func encode(_ path: String) -> String {
        path.addingPercentEncoding(withAllowedCharacters: .urlPathAllowed) ?? path
    }
    return "differati://show?old=\(encode(existing.path))&new=\(encode(failed.path))"
}

Which gives us an Xcode test failure bubble like this, with a clickable url that takes us directly into the diffing app:

Or it can be used to build a complex diff command with the path arguments aren't simply a suffix, as requested in #780:

SnapshotTesting.diffToolBuilder = { "compare \"\($0.path)\" \"\($1.path)\" png: | open -f -a Preview.app" }

[andrewjmeier]

Thank you for doing this!!!

[stephencelis]

As it stands this PR will break existing use of diffTool. We're down to support this feature (as an alternative to #508), but can you make the following change?

1. Update var diffTool to be a computed getter/setter to diffToolBuilder
2. Consider some alternative names? Maybe diffToolCommand?

[js]

Added commit with the suggested changes:

The existing diffTool always had precedence over the closure, but agreed a custom getter+setter makes the diffMessage site cleaner. However it wasn't obvious to me how to construct an empty URL for the diffTool getter so I changed the diffToolCommand closure to receive the path as String rather than the file URL.

Renamed it to diffToolCommand instead.