fix: more robust static site detection

[ascorbic]

We skip running the plugin if we detect that next export is being called. Previously we only checked if it was either the build command, or in the npm script that was called by the build command. This meant it missed build commands that were called indirectly, e.g. when the build script calls npm run export.

To fix this, I have used a package called parse-npm-script, which is designed for exactly this. However it misses a lot of edge cases, so I have vendored it and added support for parsing npm-run-all and cross-env calls. I've not done an upstream PR, because I've also removed features as I want it to be sync.

Finally, I've added support for an env var NEXT_IS_STATIC_EXPORT which can be set and will enforce this too.

Fixes #503

[lindsaylevine]

this is so awesome!! my only question is -- i was thinking the env var would be more generic. like NETLIFY_OPT_OUT_NEXT_PLUGIN or something. any reason it's so specific right now vs being an overall opt out?

[ascorbic]

Good point. I've removed the env var, as I think it needs more discussing separately from this