Add support for Symbol.getDocumentationComment

**Is your feature request related to a problem? Please describe.**

When trying to access the documentation comment on a declaration (not the jsdoc tags, just the beginning part before the tags), there doesn't seem to be any easy way to do this with ts-morph. However, the compiler node has a perfect function for this on `ts.Symbol` called `getDocumentationComment`.

For example, you could do the following:

```typescript
const typeChecker = project.getTypeChecker().compilerObject;
const docComments = symbol.compilerSymbol.getDocumentationComment(typeChecker);
const docCommentString = ts.displayPartsToString(docComments);
```

**Describe the solution you'd like**

Ideally, ts-morph would provide a way to do this natively. Something like this:

```typescript
const docComments = symbol.getDocumentationComment(project.getTypeChecker());
const docCommentString = ts.displayPartsToString(docComments);
```

**Describe alternatives you've considered**

All of the alternatives with leading/trivia comments seem to be limited in either when you can use them or how they interpret the comment text (e.g. they leave extra asterisks).

Start by tracing the ts-morph symbol API and its compilerSymbol access, then compare it with TypeScript's ts.Symbol.getDocumentationComment and the project.getTypeChecker() entry point. Done means callers can retrieve a declaration's documentation comment natively and convert the returned parts to the expected string without using compiler internals directly.

Written by the indexing model from the issue text.