Add JSDoc to document attributes for camel case properties

[web-padawan]

Description

When using lit-analyzer for linting code that uses vaadin-grid, there are warnings for certain attributes.

Expected outcome

No warnings when using attributes corresponding to camel case properties.

Actual outcome

The following warnings are shown:

    Unknown attribute 'flex-grow'. Did you mean 'flexGrow'?
    37:  flex-grow="0"
    no-unknown-attribute


    Unknown attribute 'auto-width'. Did you mean '?autoWidth'?
    47:  auto-width
    no-unknown-attribute

Steps to reproduce

1. Clone the repo: https://github.com/web-padawan/ts-vaadin-examples
2. Run yarn to install dependencies
3. Run `yarn lint:lit

Suggested fix

Add the following to JSDoc annotations for corresponding properties:

* @attr {number} flex-grow

* @attr {boolean} auto-width

[web-padawan]

Pushed a branch with some fixes https://github.com/vaadin/vaadin-grid/compare/attr-jsdoc

The problem is that @attr annotations are getting removed by gen-typescript-declarations 😕
And we need them in *.d.ts files in order to be picked up by web-component-analyzer.

[web-padawan]

Submitted a PR to gen-typescript-declarations to address that: Polymer/tools#3508
While waiting for it to land, we might create a custom build and integrate it with magi-cli.