Add support for tsconfig.json files that have an extends property

[Jtupiter]

What does the PR do?

Fixes issue #1283.

Following what was suggested in the issue to mimic what ts-node is doing to resolve the tsconfig.json here.
Their method is quite a bit more complex since they need to resolve custom ts-node compiler options.

For our purposes, reading the config file using ts.readConfigFile and then parsing it with ts.parseJsonConfigFileContent produces the expected tsconfig.json even when it extends another one.

Tests

Added a test to confirm that danger resolves extended tsconfigs.

[orta]

Hi, I'm a little surprised by the failing tests here - do you have a sense of why it broke?

[Jtupiter]

Hi, I'm a little surprised by the failing tests here - do you have a sense of why it broke?

Yeah, I was confused by this. It's completely unrelated to my change and it passes locally. The failing snapshot also looks correct, which makes me think it has something to do with line endings. Will see if I can resolve this.