Finish simplifying component definitions; component & screen generator tweaks

[bryanstearns]

Standardized component definitions

After internal deliberation at IR, we've concluded that it's best to use function syntax when declaring functional components (including screens; gets us more-useful stacktraces), and not to use React.FunctionComponent as the parent type of component prop interfaces (see #358). So, plain components are simply:

export function Foo(props: FooProps) {
  // ...
}

Components wrapped with mobx-react-lite's observer HOC (including nearly all screens) use this slightly-more-verbose version (which, yes, has the function name twice, but we feel this is a small price to pay for nice stacktraces!):

export const Foo = observer(function Foo(props: FooProps) {
  // ...
})

So, then, this PR...

I started by cherry-picking the commits from #316 which generates screens in a subdirectory; they needed merge updates against current master (thanks to @lasharela for #316, and to @BrandonSM who contributed very similar work in #331). Then:

• Fixed storybook's new warning about not configuring AsyncStorage, which we were already doing but not in a way that suppressd the warning.
• Updated the remaining predefined components, screens, and the generators to use one of those two ways of defining components.
• Modified the component generator to only generate two kinds of components, observer-wrapped and plain; the two former kinds (functionComponent and statelessFunction) didn't really make sense to me. You can avoid the question when generating by specifying --observer or --no-observer on the ignite generate component command line.
• Modified the screen generator to allow users to name the screen in CamelCase or kebab-case on the command line, and ensured that it tolerated Screen or -screen equally; it now always uses kebab-case in the filenames.

[bryanstearns]

@jamonholmgren NB: This replaces the --functionComponent and --stateless-function command line options on ignite generate component (with --observer and --no-observer); not sure whether we count that as a breaking change for versioning...

[jamonholmgren]

Waiting for #362 (cc @nirre7) and then we'll release as 6.0.0.

[nirre7]

@bryanstearns The whole function thing is "my bad". I though that to be able to use hooks one had to use the React.FunctionComponent .. Yepp, I was wrong. Thank god there are people smarter than me :relieved:

@jamonholmgren I'll try to get the #362 tests sorted today.

[infinitered-circleci]

:tada: This PR is included in version 5.3.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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