BUG - addTargetDependency - If project doesn't contain PBXTargetDependency or PBXContainerItemProxy

# Description
## One Line Summary
`addTargetDependency` misses some dependency links if `PBXTargetDependency` or `PBXContainerItemProxy` are not present.

## Affected Projects
Calls made to `addTarget` or `addTargetDependency` with any `.xcodeproj` that only has one target, these do NOT have `PBXTargetDependency` or `PBXContainerItemProxy`.

## Details
### The root cause
The following if statement in the `addTargetDependency` function skips the dependencies and other steps if it is missing.
https://github.com/apache/cordova-node-xcode/blob/8b98cabc5978359db88dc9ff2d4c015cba40f150/lib/pbxProject.js#L860

### Possible fixes
#### Option 1 - Smallest number changes
A simple way to fix this would be to use short-circuit evaluation where these `vars` are assigned to handle `null`, `undefined`, or if no key exists when reading from `hash.project.objects`.
https://github.com/apache/cordova-node-xcode/blob/8b98cabc5978359db88dc9ff2d4c015cba40f150/lib/pbxProject.js#L834-L835
This could become the following to fix this issue:
```javascript
pbxTargetDependencySection = this.hash.project.objects[pbxTargetDependency] || {},
pbxContainerItemProxySection = this.hash.project.objects[pbxContainerItemProxy] || {};
```

I have tested this on some projects on my machine and was able to successfully run an App Extension on an iOS device to confirm the solution works.

#### Option 2 - Larger Refactor - Fix some clean up
Same fix as option 1, but clean up the hard coded strings.

1. Put these at the top of the file:
```javascript
const PBX_TARGET_DEPENDECY = "PBXTargetDependency";
const PBX_CONTAINER_ITEM_PROXY = "PBXContainerItemProxy";
```

2. Use them here and remove the old vars:
```javascript
const pbxTargetDependencySection = this.hash.project.objects[PBXTargetDependency] || {};
const pbxContainerItemProxySection = this.hash.project.objects[PBXContainerItemProxy] || {};
```

## Workaround
```javascript
// Add code anytime before calling addTarget or addTargetDependency
const projObjects = xcodeProject.hash.project.objects;
projObjects['PBXTargetDependency'] = projObjects['PBXTargetDependency'] || {};
projObjects['PBXContainerItemProxy'] = projObjects['PBXTargetDependency'] || {};
```

## Related issues
PR #12 also made a note of this issue:
> 2\. `addTargetDependency()` is indirectly called as well from `proj.addTarget('AppExtension', 'app_extension')`. In my project, `PBXTargetDependency` and `PBXContainerItemProxy` sections did not yet exist which results in the necessary dependencies not being set up properly for building the extension with the main app. Create these sections if they do not exist.

They attempted to fix this in a similar way I am suggesting with commit 2e09d78efdb1d667dd513a3c97c46dc448305f5f.
This PR was not merged however, so this is still an active issue.

There is another issue #37, that references PR #12, notes that `targetDependency` is still an issue.

## Submitting a PR - Is this an active library?
It has been over a year since there has been a release or even a commit. If the maintainers could chime in I'd be happy to create one.

I have read the [CONTRIBUTING.md](https://github.com/apache/cordova-node-xcode/blob/master/CONTRIBUTING.md) guide, is there any more to it than that? I see there is a good number of tests, I'll make sure to add one to cover the code change. Lastly let me know which of the possible fixes you prefer or if you have another idea.