Support `--function` as a generator option

[simondotm]

Syntax will be nx g @simondotm/nx-firebase:function <name> --project=<projectname> [--alias=<projectname>] [--directory=<dir>] It will generate an application which is:

• based on @nrwl/node:webpack, so that the code is fully
• linked to project <projectname>
• added to the firebase.json config file functions array for that project
• assigned a codebase with the <projectname>
• it will be separately buildable and deployable
• it will have its own deploy target, which will deploy only this codebase

function applications will only work with Firebase CLI v10.7.1 and above.

[simondotm]

This neatly solves a number of other issues:

• Watched builds not building dependencies, webpack bundles everything from source
• Makes it easier to create libraries without worrying about cold start times

However, a major caveat is:

• Bundled functions include everything and this does not help cold starts
• The solution for this is to split functions into multiple apps (maybe even 1 function per app) and use codebase
• But, doing that, it's not clear just now how we'd watch multiple function projects for emulation

[simondotm]

Possibly we can add function projects as implicit dependencies of the main firebase application. Then nx watch will work.

[simondotm]

Implemented in #116