Extensionless imports in master-branch no longer supported by node-resolve

[kmmbvnr]

Hi Team!

I've tried to get working the latest master version of the material-web, and got a lot of errors like:

Missing "./decorators" export in "lit" package

    material-web/textfield/outlined-text-field.ts:10:28:
      10 │ import {customElement} from 'lit/decorators';

It seems node-resole no longer allows to import without file extension, and all imprts should be changed to import {customElement} from 'lit/decorators.js'

https://github.com/rollup/plugins/issues/684#issuecomment-770517359