npm version detection is pretty lousy

[boneskull]

Due to the deprecation of --global-style in npm v9, #242 introduced a version check to avoid triggering the warning. However, the version check just looks at the first character of the output from npm --version and uses --global-style if it is 7 or 8.

Given npm v6 and older is unsupported, I'm not sure we need to worry about it. But it could result in a misleading error message if that's indeed the case. Nevermind that this will break again when npm releases version 70. 😝

I wanted to avoid pulling in all of semver to check the version. Maybe there's something smaller and more robust we could use.

[boneskull]

closing given the plugin-like refactor of the package manager code