Make alt prop required

[Lehoczky]

For accessibility reasons every image element should have an alternative text.

There are multiple code analyzer tools enforcing this, for example htmlhint, angular-eslint and eslint-plugin-vue-accessibility. It would be better if developers saw errors without using one of these (or other) tools and got an error directly from the image component.

For reference, the image component for NextJS and astro requires the alt text. Quotes from their docs:

The new Image component:

• Ships less client-side JavaScript
• Easier to style and configure
• More accessible requiring alt tags by default

https://nextjs.org/blog/next-13#nextimage

For accessibility, the required alt prop helps improve your site for screen-readers and other tooling.

https://astro.build/blog/images/

What do you guys think about this change?

[Lehoczky]

Welp sorry, duplace of #315