feat: add cem analyzer

[bennypowers]

closes #48

What I did

1. add cem/a as a devdependency in common
2. add analyze script in common
3. add customElements package.json key in common
4. add custom-elements.json to gitignore in common
5. add analyze to build and publish steps in various packages
6. add scriptRunCommand to all templates
7. analyze the generated package after installing deps
8. add test
9. require node >= 14
10. add --help switch
11. remove "extends" docs from README (see docs: add "extending" docs to generator page open-wc#2270)
12. add args table to README

TODO

• tests

--help output

        _.,,,,,,,,,._
     .d''           ``b.       Open Web Components Recommendations
   .p'      Open       `q.
 .d'    Web Components  `b.    Start or upgrade your web component project with
 .d'                     `b.   ease. All our recommendations at your fingertips.
 ::   .................   ::
 `p.                     .q'
  `p.    open-wc.org    .q'
   `b.     @openWc     .d'
     `q..            ..,'      See more details at https://open-wc.org/init/
        '',,,,,,,,,,''



Usage

  $ npm init @open-wc [<options>]

Options

  --destinationPath path                        The path the generator will write files to
  --type scaffold|upgrade                       Choose scaffold to create a new project or upgrade to add features to an
                                                existing project
  --scaffoldType wc|app                         The type of project to scaffold. wc for a single published component, app for
                                                an application
  --features linting|testing|demoing|building   Which features to include. linting, testing, demoing, or building
  --typescript true|false                       Whether to use TypeScript in your project
  --tagName string                              The tag name for the web component or app shell element
  --installDependencies yarn|npm|false          Whether to install dependencies. Choose npm or yarn to install with those
                                                package managers, or false to skip installation
  --writeToDisk true|false                      Whether or not to actually write the files to disk
  --help                                        This help message

Questions

• should the step which analyzes the package after install happen in every case (e.g. via npx)? even if not installing?
• under this pr, analyzer is not optional, everyone always gets it. cool?

[bennypowers]

@thepassle fails in node 12 b/c of optional chaining in cem/a
so either we bump requirements to 14 or build/refactor cem/a, right?

[thepassle]

Is there any particular reason why we use node 12? I think we can just upgrade right?

Im really averse to adding a build step to cem/a

[bennypowers]

upgrading means requiring users to be on node 14

• firebase supports node 14
• gcp supports node 14
• azure supports node 14
• aws supports node 14
• heroku uses 14 by default
• qovery ???
• vercel uses 14 by default
  I'm super happy to make this breaking change. @LarsDenBakker ?

[bennypowers]

⚠️ ⚠️ ⚠️ WARNING ⚠️ ⚠️ ⚠️ : either don't squash, or make sure the merge commit is a feat!: