Upgrade to TypeScript 2.4

[kitsonk]

We need to upgrade to TypeScript 2.4.

Considerations:

• Weak Type detection will likely cause some regressions in the typings where we have been loose, or might highlight some issues with inaccurate or unsafe code. There are also several other changes that will cause generics to be inferred better which may allow us to type things better, but also might cause regressions.
• Migrate to ES import() - This should be able to fully replace our load() usage.
• Adopt string enums as appropriate

Here are the repos we need to consider:

• - dojo/loader
• - dojo/has
• - dojo/shim
• - dojo/core
• - dojo/interfaces
• - dojo/widget-core
• - dojo/widgets
• - dojo/routing
• - dojo/dgrid
• - dojo/i18n
• - dojo/cli
• - dojo/cli-build
• - dojo/cli-test-intern
• - dojo/cli-create-app
• - dojo/cli-export-project

[kitsonk]

dojo/has had no regressions