add conditional check to block javascript: URLs

[gtsp233]

Fix for Cross-Site Scripting (XSS) Vulnerability

I've identified a Cross-Site Scripting (XSS) vulnerability in this package.

Vulnerability Details:

• Severity: High/Critical
• Description: There's a risk of malicious script execution when the href of the a element is controlled by an adversary.

Steps to Reproduce:
In a React.js project:

import { Link } from 'gatsby-theme-advanced'

<Link href={`javascript:alert(1)`} />

Then the malicious code alert(1) will be executed.

Suggested Fix or Mitigation:
It is best practice for a React.js components package to sanitize the href attribute before passing it to an tag. React.js itself, along with many popular libraries such as react-router-dom and Next.js, also ensures the safety of href attributes. For instance, React.js issues warnings about URLs starting with javascript: and is planning to block these in future versions, as indicated in this pull request.

I've already fixed and tested this issue, and have submitted a pull request with the necessary changes. Please review and merge my pull request at your earliest convenience to resolve this vulnerability. Thanks!

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