Add GH action to prevent changes to Extensions list

[heitortsergent]

The k6 Extensions explore.md file includes a list of k6 supported extensions, and it's automatically updated by a GH action whenever changes are made to the k6 extension registry.

We should add a GH action that prevents changes from being made to that file manually, to avoid those two sources from getting out of sync and prevent confusion from docs maintainers.

Example GH action: https://github.com/marketplace/actions/prevent-file-change