pbxProject.prototype.addFramework: automatically create Frameworks Group if it does not exist

**This is a feature proposal**

**Initial Situation**:

Your xcode project tree looks like this:

You have a Xcode project without a `Framworks` group and you want to add a new framework using `pbxProject.prototype.addFramework`.

**Current behaviour**:

The `Frameworks` group is created but not added to the `mainGroup` of the project which references all files (that are not part of a subgroup) and all groups (that are no subgroups).

If you open the updated xcode project with xcode you see the following:

This is because xcode cannot find a reference to the `Frameworks` group on the `mainGroup` and therefore "autofixes" the project by adding the library added with `pbxProject.prototype.addFramework` to a newly generated group `Recovered References`.

**Expected behaviour**:

The `Framework` group is created and also automatically added to the `mainGroup`.

When you open the project in xcode it should look like this:

**Current workaround:**

Add the `Frameworks` group to `mainGroup`manually:

```js
const project = xcode.project(projectPath);
project.parseSync();

project.addFramework(path.resolve(frameworksDirectory, 'GoogleInterchangeUtilities.framework'), {
customFramework: true,
embed: true,
link: true,
});

const mainProject = project.getPBXGroupByKey(project.getFirstProject().firstProject.mainGroup);
if (!mainProject.children.some(children => children.comment === 'Frameworks')) {
const [groupKey] = Object.entries(project.hash.project.objects['PBXGroup']).find(
([, group]) => group.name === 'Frameworks',
);
project.addToPbxGroup(groupKey, project.getFirstProject().firstProject.mainGroup);
}
```

If you are okay with this feature proposal I would love to implement it!

从 pbxProject.prototype.addFramework 开始，跟踪 Frameworks 组和 project mainGroup 的处理方式。使用 issue 中的 workaround 作为行为参考。当缺失的 Frameworks 组既被创建又被 mainGroup 引用，从而使 Xcode 不将 framework 放入 Recovered References 时，即表示完成。

由索引模型根据 Issue 内容生成。