Add declarationMap to allow users to jump to source

[jvliwanag]

Tanstack router goes through lengths to be typesafe. However, we end up with rather complicated types that's a bit hard to follow. On an app using tanstack router, it'll be particularly helpful to jump to the ts source directly with an lsp supported editor.

Looks like projects within this repo already publishes ts sources. The only missing piece is to enable declarationMap on the tsconfig and publish those mappings.

[jvliwanag]

Actually, might have oversimplified the solution. Forgot that it's not typescript that's used to build.