Use an `app` root object for all function registrations

[ejizba]

Collecting feedback from various places on the app root object. Seems like everyone is overwhelmingly in favor of it, but we can have further discussion here.

From @aaronpowell:

• I like this approach as it mirrors the kind of thing we find with Express, Fastify, and other web frameworks
• It also makes for a clear ordering to the Functions that are defined, so if you've got multiple HTTP triggers that work off similar routes, you should have a clear order of precedence
• It would also make it easier to have discovery, rather than having to know what you can import, you import one thing and intellisense will tell you want it can do

From @YunchuWang

With a central object like app for managing functions, it is more intuitive for users to know/look up methods compared to option1 via intellijsense. (ex, typing app. autocompletes list of methods)

[ejizba]

Closing in favor of Azure/azure-functions-nodejs-library#12