[tsconfig] Investigate using declarationMap to make source jumping easier

[kirbysayshi]

https://stackoverflow.com/a/51261384/169491

Apparently adding declarationMap to the declaration generation would allow for things like "Go To Definition" to take you to the actual TS instead of the type declaration when using a web-scripts-built module from another project!

[fastfrwrd]

Since it's for every .d.ts file per the compiler options doc, it only needs to be added to the types build, right?

[kirbysayshi]

I believe so! But I haven't played around with it yet.

[fastfrwrd]

works like a charm!