FD-101194 - Add support for tsconfig.json files that have an extends property

[Jtupiter]

When we tried to run danger with an extended tsconfig.json we ran into issues. Tracked it down to this issue on the danger-js repo danger#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.