Introduce a New Matcher: toHaveTagName

[porada]

What:

This PR implements a new matcher, toHaveTagName, useful for testing polymorphic components.

Why:

expect(myComponent).toHaveTagName('section');

// Much more elegant and expressive than
//
// expect(myComponent.tagName).toStrictEqual('SECTION');

Checklist:

Right now, this is still work in progress. Apart from the checklist below, the feature is currently missing the support for non-lowercase SVG elements and custom elements, plus test coverage.

If there’s consensus around including this matcher, I’ll be happy to finalize it.

• Documentation
• Tests
• Updated type definitions
• Ready to be merged

[porada]

@gnapse — I’ve been using this custom matcher in my codebase and wanted to formalize it here. Based on past discussions, I believe you expressed interest in adding toHaveTagName at some point. Let me know if I have your green light to finalize the implementation.