[request] Append `gitHead` to package.json

[teppeis]

Currently published @​types packages don't include git revision of DefinitelyTyped repo, so we cannot get to know the exact diff between previous version and new one without actually comparing downloaded packages.

npm publish appends gitHead field to package.json: npm/read-package-json#7

I think the field is very useful for @​types packages.

[teppeis]

Usecase: automated dependency updates service like Renovate

It creates PR to update deps with changelog: ex) teppeis/bind.ts#31
But DefinitelyTyped repo doesn't have git tag for each package and @​types package doesn't have gitHead prop, it cannot generate changelog: ex) teppeis/bind.ts#42

[felixfbecker]

We also need this property at Sourcegraph to provide go-to-definition to the DefinitelyTyped repository at the correct revision.