feat: Add support for Custom tsconfig.json, Resolve extended configs

[Saeris]

This addresses both oclif/oclif#299 and oclif/oclif#488

loadTSConfig() in src/ts-node.ts was not properly resolving extended configuration files, because while parseConfigFileTextToJson will load and parse a given tsconfig.json file, it does not merge compilerOptions from configs specified in the extends option.

This implementation is based on the solutions suggested in microsoft/TypeScript#5276
and https://stackoverflow.com/questions/53804566/how-to-get-compileroptions-from-tsconfig-json which rely on Typescript's utilities for loading and parsing tsconfig.json files.

Specifically, by adding a call to parseJsonConfigFileContent(), the merged compilerOptions will be resolved as expected.

Additionally, this adds support for user-defined tsconfig files through the tsConfig option under oclif key in their package.json.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @Saeris to sign the Salesforce.com Contributor License Agreement.

[Saeris]

I'm not sure what is causing the tests in https://github.com/oclif/config/blob/master/test/typescript.test.ts to fail. Some guidance here would be appreciated.

[Saeris]

Should also be noted that I bumped typescript up to the latest version. This facilitates importing the official CompilerOptions type definition. It should also allow for ts-node to compile the latest supported syntax/typescript features. In my own development with Oclif I've noticed that it won't compile commands that use Optional Chaining syntax. I haven't tested whether this change resolves that specific issue.

[AKK9]

Is oclif actively maintained?