fix(webcomponents): make types play nicely with lit-element

[urish]

What I did

Before this change, returning html<sometag>...</sometag> from storiesOf(...).add would result in the following error: Type 'TemplateResult' is not assignable to type 'StoryFnHtmlReturnType'

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[ndelangen]

I think we need a eslint-ignore-next-line rule for:

/tmp/*********/app/web-components/src/client/preview/types.ts
  2:1  error  'lit-element' should be listed in the project's dependencies. Run 'npm i -S lit-element' to add it  import/no-extraneous-dependencies

[urish]

@ndelangen do you want me to amend the pull request and add this?

[ndelangen]

I read: "would you please amend the PR", so I went ahead and did it.

Thanks for the PR @urish!

[urish]

Even better :-)

p.s. you can see it in action here: https://elements.wokwi.com/
source code: https://github.com/wokwi/wokwi-elements