".npmrc" are configuration files used by "npm", which in turn is a
widely used package manager in web-development setting (javascript,
typescript, etc).

I don't have much expertise in NPM field, but judging by the official
documentation¹ it's basically ini-style `key = value`, but with two
comment styles (semicolon and hash sign).

Easily implementable by deriving from conf-mode and adding the comment
styles.

Patch is attached.

1: https://docs.npmjs.com/cli/v11/configuring-npm/npmrc