feat: Add privatePrefix for private environment variables

[elliott-with-the-longest-name-on-github]

Closes #9776.

Adds a new config option, config.kit.env.privatePrefix, for setting a private prefix on environment variables. This defaults to ''.

To prevent super-weird and unexpected behaviors, the logic is:

• private: does not begin with public prefix, does begin with private prefix
• public: does begin with public prefix, does not begin with private prefix

This has the side benefit of not allowing the two prefixes to be the same.

Note: Had to create env.js utils file so that the utils could be imported into server/index -- putting them pretty much anywhere else ended up causing a transitive dependency on some node package somewhere that wasn't compatible with Node 16.

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[benmccann]

just a heads up, there's a merge conflict on this one

[elliott-with-the-longest-name-on-github]

Weird; thanks