Linter: Implement html-no-style-attributes and html-no-style-elements rules

[markokajzer]

closes #162

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Rule: html-no-style-attributes

Description

Disallow the use of inline style attributes on HTML elements. All styling should be applied via external stylesheets or class-based styling.

Rationale

Inline style attributes make templates harder to maintain, promote duplication, and prevent adoption of strict CSP policies (style-src 'self'). Using CSS classes and external stylesheets improves maintainability and security.

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Rule: html-no-style-elements

Description

Disallow the use of inline <style> tags in HTML templates. All styling should be applied via external stylesheets.

Rationale

Inline <style> tags make templates harder to maintain, promote duplication, and prevent adoption of strict CSP policies (style-src 'self'). Centralizing styles via external stylesheets improves maintainability and security.

This rule enforces:

• No <style> tags embedded directly in templates.
• No style tags generated via content_tag or tag helpers.

[marcoroth]

Thank you for working on this @markokajzer! 🙏🏼

Similar to #1536 (review), I think we also want to rename this rule to have a html-* prefix and have it split up to two rules, something like:

• html-no-style-attributes: no style=""
• html-no-style-elements: no <style>...</style>

[markokajzer]

changes:

1. split out html-no-style-attributes
2. split out html-no-style-elements
3. both scan for action view helpers
4. added some additional specs with more edge cases

i can separate into two PRs 👍