Linter: Implement `a11y-image-has-alt` rule

[marcoroth]

Implement the GitHub::Accessibility::ImageHasAlt rule from erblint-github.

Rule Details

<img> should have an alt prop with meaningful text or an empty string for decorative images.

Examples

Incorrect code for this rule:

<img src="logo.png">

Correct code for this rule:

<img alt="" src="logo.png">
<a href="https://github.com/"><img alt="GitHub homepage" src="logo.png"></a>

References

• erblint-github source
• erblint-github docs
• W3C WAI Images Tutorial
• Primer: Alternative text for images

Parent Issue

• erblint-github compatibility/parity #1205

[marcoroth]

Resolved via #1311 and the existing html-img-require-alt rule from #171