Add support for :host pseudo-class

[RedZapdos123]

Description:

• Add support for :host by aliasing it to :scope.
• Add regression tests that verify :host matches the root element and context elements.
• Document :host support in the README selector list.

Verification:

• npm run test:vi
• npm run build
• node --input-type=module -e "import * as CSSselect from './dist/index.js'; import { parseDocument } from 'htmlparser2'; const dom = parseDocument('
  ').children; console.log(CSSselect.selectAll(':host', dom).length);"

Fixes #1374

The screenshot of the tests run, for validation:

Summary by CodeRabbit

• New Features
  • The :host pseudo-class selector is now supported. It selects elements in the current context and operates as an alias to :scope, offering an alternative syntax for context-aware selection.

[coderabbitai]

📝 Walkthrough

Walkthrough

Added support for the :host pseudo-class by creating an alias to :scope. Changes include documentation updates, a new alias mapping in the pseudo-selectors module, and corresponding tests verifying root element and context matching behavior.

Changes

Cohort / File(s)	Summary
Documentation README.md	Updated "Supported selectors" section to document :host as an alias of :scope.
Implementation src/pseudo-selectors/aliases.ts	Added new alias mapping: host: ":scope".
Testing test/pseudo-classes.ts	Added test suite for :host pseudo-class verifying root element selection and context-aware matching.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A little :host hops into place,
As :scope's trusty alias in the race,
Through DOM trees it bounds so free,
Selecting context, one, two, three! 🌿

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR successfully addresses issue #1374 by implementing :host pseudo-class support, adding regression tests, and documenting the change in the README.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing :host support: the alias mapping, tests, and documentation. No unrelated modifications are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title 'Add support for :host pseudo-class' directly and clearly summarizes the main change in the pull request, which adds :host pseudo-class support.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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