A11y warnings for usage of placeholder

[theetrain]

Describe the problem

Too often are placeholder attributes used instead of <label>. Let's provide some warnings to steer developers into proper usage.

Describe the proposed solution

Criteria

• When label is not present (implicitly wrapped, or explicitly with for), show warning when placeholder is used on its own

Reference: https://www.a11yproject.com/posts/placeholder-input-elements/

Importance

nice to have