Add multiline string diffs

[1Jajen1]

This lays out multiline strings line by line and compares them, producing a much nicer diff on failure.

A test like this:

"Hello\nWorld\n?".eqv("Hello\nWelt\n?!")

Produces:

- Hello
 World
 ?
+ Hello
 Welt
 ?!

But now with this pr it produces:

  1. Hello
- 2. World
+ 2. Welt
- 3. ?
+ 3. ?!

This is only enabled for strings that contain \n. All others will still render on top of each other without line numbers.

[1Jajen1]

Thanks @pakoito, now to figure out why the build did not run...

[1Jajen1]

Btw the description is not 100% accurate:
Because the algorithm for diffing is a ses algorithm the diff will not be line by line.

This examples shows this better:

"Hello\nWOrld\n\n.".eqv("Hello\nWorld\n!\n.")
  1. Hello
- 2. WOrld
- 3.
+ 2. World
+ 3. !
  4. .

This is very useful if long multiline texts change by removing or inserting a newline in the middle, however this sucks for short multiline text where I'd prefer:

  1. Hello
- 2. WOrld
+ 2. World
- 3.
+ 3. !
  4. .

Both things are super easy to implement and I don't really have a strong opinion on either. (The current one is just nice because it reuses list diffing).

Thoughts?