Add `project.items.addByContentId` and `project.items.addByContentRepositoryAndNumber`

### Please avoid duplicates

- [X] I checked [all open feature requests](https://github.com/gr2m/github-project/issues?q=is%3Aissue+is%3Aopen+label%3Afeature) and none of them matched my problem.

### What’s missing?

We have `project.items.updateByContentRepositoryAndNumber` and `project.items.removeByContentRepositoryAndNumber`, but we do not have a method to add an existing issue or pull request using its repository name and number.

(I'm not sure if using `by` is grammatically correct English, I'm open to alternative ideas. But the names would be consistent with it's `project.items.update*` and `project.items.remove*` counterparts that way.)

This would replace the current `project.items.add`

### Why?

see ☝🏼

### Alternatives you tried

manually retrieve an issue/PR object in order to retrieve the node ID, then pass it to `project.items.add(nodeId)`

### Would you be interested in contributing the feature?

- [ ] yes