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

Start by reading the existing project.items.add, project.items.updateByContentRepositoryAndNumber, and project.items.removeByContentRepositoryAndNumber methods to understand their public API and naming. Done means project.items.addByContentId and project.items.addByContentRepositoryAndNumber can add an existing issue or pull request without first retrieving its node ID, while keeping the related behavior consistent.

Written by the indexing model from the issue text.