[api-extractor] Surface whether a class member is protected

[zelliott]

Summary

Consider the following class:

class Example {
  protected name: string;
  protected constructor(name: string) {
    this.name = name;
  }
  protected doThing(): void {}
}

In all three of the types of members above (i.e. property, method, constructor), class members can be marked as protected. This information isn't included in the .api.json file in a structured way (besides the protected keyword existing in the item's excerptTokens field).

We should expose an isProtected property on the ApiProperty, ApiMethod, and ApiConstructor items. We can do this by adding a new ApiProtectedMixin mixin (or ApiVisibilityMixin?) that functions similarly to the existing ApiStaticMixin.

Details

N/A

Standard questions

Please answer these questions to help us investigate your issue more quickly:

Question	Answer
@microsoft/api-extractor version?	Latest
Operating system?	All
API Extractor scenario?	docs
Would you consider contributing a PR?	Yes
TypeScript compiler version?	N/A
Node.js version (node -v)?	N/A

[octogonz]

Thanks for making a PR!