Extend xcdiff to become a git difftool for Xcode Projects

**Is your feature request related to a problem? Please describe.**

`xcdiff` can diff two standalone Xcode Projects. An interesting use case that can be extended from this is using `xcdiff` as a git difftool to allow comparing an Xcode project against itself within source control.

**Describe the solution you'd like**

There a few bits needed to make this work:

- Support diffing `.pbxproj` files directly (not just `.xcodeproj`)
- Difftools are passed two files to diff, in for Xcode project files this will be the underlying `.pbxproj` files
- Add a new renderer that uses terms like "Removed" / "Added" rather than "Only in first" / "Only in second"
- Optionally this renderer could use terminal colors, green text for added items, red for removed, yellow for modified

- Update documentation with instructions on how to add `xcdiff` as a diff tool

e.g.

```sh
git config --local difftool.xcdiff.cmd 'xcdiff -p1 $(dirname "$LOCAL") -p2 $(dirname "$REMOTE") -d -v
```

```sh
git diff --tool xcdiff
```

_There might be a way to make git automatically select a diff tool for certain file extensions which we could leverage as well (needs more research)._

**Describe alternatives you've considered**

Instead of extending `xcdiff` itself, we can leverage a bash script to marshal the `.pbxproj` files to the desired format that enables `xcdiff` to read it.

e.g.

```sh
#!/bin/bash
tmp_dir_1="$(mktemp -d -t xcdif)"
cp "$1" $tmp_dir_1/project.pbxproj

tmp_dir_2="$(mktemp -d -t xcdiff)"
cp "$2" $tmp_dir_2/project.pbxproj

xcdiff -p1 "$tmp_dir_1" -p2 "$tmp_dir_2" -d -v
```

Another alternative could be to add an additional executable target `xcgitdiff` that manages some of this under the hood and uses a special renderer that is more suitable for this use case.

**Additional context**

I have a [prototype](https://github.com/kwridan/xcdiff/commits/feature/gitdiff) of a custom renderer that replaces terms:

- "Only in first" > "Removed"
- "Only in second" > "Added"
- "Value mismatch" > "Modified" or "Changed"

`xcdiff --format git`

Not sure how useful this feature is, but posting here to collect ideas and see if there's interest to pursue this further.

Start by reviewing xcdiff's existing project-input and renderer paths, then compare them with the feature/gitdiff prototype mentioned in the issue. Check how direct .pbxproj inputs and the proposed git difftool commands should flow through the tool. Done means direct project-file diffing, a suitable added/removed/modified renderer, and documented setup instructions.

Written by the indexing model from the issue text.