--showConfig just like tsc, to help users inspect the config, see what --files is doing

[cspotcode]

ts-node --showConfig can emit output similar to tsc --showConfig, displaying ts-node's configuration: both ts-node flags and tsconfig flags, emitted in tsconfig format with our nested "ts-node" object.

Looks like TypeScript does it like this:

sys.write(JSON.stringify(convertToTSConfig(configParseResult, configFileName, sys), null, 4) + sys.newLine);

https://github.com/microsoft/TypeScript/blob/master/src/executeCommandLine/executeCommandLine.ts

Unfortunately convertToTSConfig is @internal, so we will need to be confident that it won't break. It was added in TS 3.2; I don't know if the function signature has stayed the same.

We'll also need a ts-node equivalent of convertToTSConfig which can generate our ts-node sub-object. I think it should mostly be verbatim, but some things like paths will need to be computed correctly.