[6.x] Add .npmrc file

[duncanmcclean]

This PR adds an .npmrc file with two rules:

• Prevents post-install scripts from running, which are commonly used to install malware.
• Adds a minimum release age of 3 days to provide sufficient time for insecure releases to be found and pulled if necessary (like what happened with axios recently).

Adding as I've noticed Laravel adding these rules to a lot of their repositories recently and thought it'd be a good thing for us to adopt too.