Add Monkeytype username validator module

[articulatebass]

This pull request adds a Monkeytype username availability checker to the gaming category.

The module queries the official Monkeytype API:

https://api.monkeytype.com/users/checkName/<username>

It follows the same return pattern as the other platform modules:

• 1 → Available
• 0 → Taken
• 2 → Error / Uncertain

The module has been tested locally using the user-scanner CLI.

[kaifcodec]

@articulatebass Thanks for the PR, it looks clean and aligns well with the contribution guidelines. I’m a bit busy at the moment, but I’ll review the code and test it locally as soon as I get some free time. Really appreciate your work!

In the meantime, feel free to take a look at issues #12 and #13, or explore my other repositories and tools like ytconverter

[kaifcodec]

Thank you @articulatebass for showing your effort towards user-scanner.
Really appreciate your work.