[dev] Add support for injecting shared environment variables

[eddiecooro]

Fixes #431

[erquhart]

Thanks for opening this @Eddie-CooRo!

@RaeesBhatti the approach here seems reasonable - I'll defer to you on review in case there's a non-obvious issue. Would be worth adding some testing around this and existing env var injection, too, but no need to block this PR for it.

[eddiecooro]

Hey @erquhart thanks for your reply
@RaeesBhatti have you seen any issue in this PR? We upgraded our account to pro just because of shared env variables, and netlify dev not supporting it is a little troublesome for us. because of this, I have to use a custom build of this package for our team, and using the awesome features you guys are working on, needs some rebasing and dirty works.
It would be great if we could have this feature in the official package.
thanks in forward.

[RaeesBhatti]

Hey @Eddie-CooRo, thanks for opening this PR. It took longer than it should've but we've confirmed the intended behavior.