Add ability to open menu items in a new tab

[cmd05]

What does this PR change? What problem does it solve?

Add the ability to open specific menu items in a new tab. When an identifier begins with "ext-", it will add a target="_blank". This is independent of whether the link is an external link, so the user can decide themselves. For example, if I want to make my blog page open in a new tab:

menu:
  main:
    - identifier: "ext-blog"
      name: Blog
      url: /blog/
      # or url: https://blogsite.com 
      weight: 1

Was the change discussed in an issue or in the Discussions before?

Inspired by this PR

This discussion to open all external links in a new tab. Hugo still has not implemented target parameter for menu items. Issue

PR Checklist

• [ ] This change adds/updates translations and I have used the template present here.
• [x] I have enabled maintainer edits for this PR.
• [x] I have verified that the code works as described/as intended.
• [ ] This change adds a Social Icon which has a permissive license to use it.
• [x] This change does not include any CDN resources/links.
• [x] This change does not include any unrelated scripts such as bash and python scripts.
• [ ] This change updates the overridden internal templates from HUGO's repository.

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rickeygong]

Thank you