Add MAC Address Generator

[nick-alltoolsverse]

Adds MAC Address Generator to the Online tools section.

The browser-local utility generates random test addresses in colon, hyphen, or Cisco dotted notation, with uppercase and lowercase output plus a locally administered bit option.

Validation:

• Generated four unique uppercase addresses in Cisco dotted notation.
• Confirmed every address had the locally administered bit set and the multicast bit clear.
• Confirmed the generator uses browser cryptographic randomness and performs no network request during generation.
• Searched the README and pull request history for duplicate entries.
• Added one canonical URL and one line at the end of the section, as required.

Disclosure: I am the maker of All Tools Verse. OpenAI Codex assisted with repository research, live testing, and preparing this pull request.