feat: prefer ideal image

[Azurinine]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

This PR address the concerns brought up in issue #6472, and enforces the use of @theme/IdealImage component instead of tags.

Following feedback from @slorber in previous discussions (#8826, #11416), this rule adopts a tiered approach to minimize false positives and noise:

• Error Tier: Triggered for src={require('./img.png')}. Since the asset is explicitly required, we are certain it is a local file that would benefit from the Ideal Image optimization pipeline.
• Warning Tier: Triggered for static relative paths (e.g., ./img.png) or root-relative paths (/img.png). This nudges the user to optimize if the file is indeed local.
• Silent/Ignored: * External URLs (https://...) and protocol-relative links (//).
  • Dynamic values (e.g., src={props.src} or src={someVariable}) to avoid annoying users in generic components.
  • SVGs: Since IdealImage does not support vector formats, these are explicitly ignored to prevent useless warnings.

Test Plan

I have implemented a comprehensive test suite in prefer-ideal-image.test.ts covering the following:

• Valid cases: External HTTPS links, template literals with external prefixes, dynamic props/variables, and .svg files.
• Invalid cases: Standard <img> tags using require(), relative strings, and manual srcset implementations.

Tested locally using yarn jest packages/docusaurus-eslint-plugin with all cases passing.

Related issues/PRs

• Fixes Create ESLint plugin rules to enforce best Docusaurus practices #6472 sub-issue
• Credits to @Devansu-Yadav: this PR builds on the logic and feedback from Add new prefer-ideal-image eslint rule #8826.
• Incorporates feedback from feat(eslint): add prefer-ideal-image rule #11416 regarding noise reduction and ambiguity.

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