New —declarationMap

Enabling —declarationMap alongside —declaration causes the compiler to emit .d.ts.map files alongside the output .d.ts files.Language Services can also now understand these map files, and uses them to map declaration-file based definition locations to their original source, when available.

In other words, hitting go-to-definition on a declaration from a .d.ts file generated with —declarationMap will take you to the source file (.ts) location where that declaration was defined, and not to the .d.ts.