Fluentui: [DevExp] Add declaration mapping so that F12 navigation from one pkg to another goes to the src

[kenotron]

Describe the feature that you would like added
Currently in VS Code, because we only do declaration: true, navigating code from one src file to another goes to the declaration even though we're all in a single monorepo. We should generate declaration map to navigation from src to src.

[kenotron]

Shoot, we don't minbar TS2.9: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html

We cannot do this until Fabric 7.0.

[cliffkoh]

@micahgodbolt @kenotron should this be added to some 7.0 milestone?

[micahgodbolt]

if it's a 2.9 minbar, I think 7.0 is definitely appropriate.