Main feature:
- Adding a new html format `htmlSideBySide` that displays results in a diff like format
- This can sometimes be easier to read through, espically when comparing differences in common values

Supporting changes:
- HTML escpae utility was pulled out to its own extension to allow sharing between both html renderers
- A small fix was made to the Makefil to enable regenerating tests

TODO:

- [ ] Iterate on styles, are there any better options?
- [ ] Are there any alternate format names that are more suitable (e.g. `htmlDiff`)?

Test Plan:

- Generate a diff using the new side by side format and examine the results

```sh
swift build
LOCAL_XCDIFF=$(pwd)/.build/debug/xcdiff

cd Fixtures
$LOCAL_XCDIFF --format htmlSideBySide --verbose > results.html

open results.html
```

Signed-off-by: Kassem Wridan <[email protected]>

- Added grey background color for empty cells
- Added a different yellow color for value mismatch keys
- Removed "Value mismatch" label and replaced with a spacer

Signed-off-by: Kassem Wridan <[email protected]>