make TsConfigResolver._parseJsonConfigFileContent() function public

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

Hey @dsherret! Awesome project, thank you :bow:!
I'm working on a small package that would help rename specifiers on-the-fly when using multiple TS compilation targets.
To create the TS Program I need to support project references, but currently the `TsConfigResolver` class does not expose those.

Hence, to recreate the tsProgram correctly I have to write this code that uses the private method `tsConfigResolver._parseJsonConfigFileContent()`:

```ts
const tfsHost = new TransactionalFileSystem({
fileSystem: fsHost,
skipLoadingLibFiles: true,
libFolderPath: undefined,
});

const standardizedTsConfigPath =
tfsHost.getStandardizedAbsolutePath(tsConfigFilePath);

const encoding = "utf-8";
const tsConfigResolver = new TsConfigResolver(
tfsHost,
standardizedTsConfigPath,
encoding,
) as unknown as PrivateTsConfigResolver;
// ^^ need to cast this to include the private method

const tsConfigContent = tsConfigResolver._parseJsonConfigFileContent();

const tsProgram = ts.createProgram({
options: tsConfigContent.options,
configFileParsingDiagnostics: tsConfigContent.errors,
projectReferences: tsConfigContent.projectReferences,
// ^^ here
rootNames: [],
});
```

**Describe the solution you'd like**

Ideally, the TsConfigResolver would expose the full config contents. Currently it only exposes these two (`getCompilerOptions`, `getErrors`):

https://github.com/dsherret/ts-morph/blob/930779a0d3699be35ab17bbe9dd26ec00350fe53/packages/common/src/tsconfig/TsConfigResolver.ts#L21-L27

Either adding a public method like `getParsedCommandLine()` or exposing `parseJsonConfigFileContent()` as public would solve my problem.

**Describe alternatives you've considered**

Alternative I've considered is to copy and paste a bunch of code from ts-morph and including that in my project. Not ideal, since I'd need to sync that every time the upstream changes.