refactor: use getLocFromIndex in no-html

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This PR originated from #536. While working on that, I found the scope had become too large, so I split this refactoring into a separate PR.

In this PR, I've used the native getLocFromIndex method added in #376 instead of the custom findOffsets util.

Notable changes:

• Replace the findOffsets util with the native getLocFromIndex.
• Use a named capture group in htmlTagPattern for better readability.
• Consolidate the tag-name normalization logic (allowedIgnoreCase ? tagName.toLowerCase() : tagName) into a single helper function, normalizeTagName.
• Remove the unnecessary allowedElements.size === 0 check from the if clause when reporting.

What changes did you make? (Give an overview)

In this PR, I've used the native getLocFromIndex method added in #376 instead of the custom findOffsets util.

Related Issues

Ref: #376, #536

Is there anything you'd like reviewers to focus on?

N/A

[mdjermanovic]

LGTM, thanks!