This introduces a dependency on Spina, our modern wrapper around

Backbone (https://www.npmjs.com/package/@beanbag/spina). Spina can now

be used to develop JavaScript/TypeScript in Djblets.
There's a NPM dependency on Spina, enabling type checking and module

management, but Spina itself is not bundled in Djblets. It will be up to

any consumers to embed both Backbone (as normal) and Spina in their

trees.
tsconfig.json is also set up to use Spina's version of the Backbone

type definitions for TypeScript, which feature some fixes and enable

some features not present in @types/backbone.

Put some test Spina code in a file and verified that the module compiled

and I could access and instantiate the object within the JavaScript console.
Verified I could build packages with this.