Core: Update diff library, integrate with TapReporter

[Krinkle]

We embed an old version of the diff-match-patch library for use in the HTML Reporter. The latest version has some improvements that I think reduce noise, and its line-based text mode (i.e. not HTML tags) makes it suitable for CLI use, so we can integrate it in the TAP reporter.

Our wrapping functions are at the end of src/core/diff.js.

It seemed like the library has built-in methods to generate a unified diff, but I couldn't get it to work well in practice, so I'm reducing it to a line-blocks first and adding plus/minus symbols manually in our wrapping function. Perhaps there's a better way?

Ref #1412.

Before	After

• More reliably skip diff when it's only "plus all of expected, minus all of actual".
• The updated version of diff-match-patch has slightly better semantic/word thresholds.
  It thus creates a slightly different balance between making it easy to spot subtle differences (diffing the exact character, such as adding an s or missing a 1 in the above screenshot), and making it easy to understand larger meaningful changes (such as diffing helloo and goodbye as whole words instead of treating the oo as semantic overlap and diffing individual letters).