Add REACT_APP_INJECT_STYLES to allow injecting generated styles into window variable

[rileytb]

This change allows REACT_APP_INJECT_STYLES to control injecting styles into a window variable (which is then used within hf-inj-react to inject them into the shadow DOM for the zion header/footer)
The use of style-loader for prod builds when REACT_APP_INJECT_STYLES is enabled - instead of MiniCssExtractPlugin is because of the warning under this section: https://webpack.js.org/plugins/mini-css-extract-plugin/#insert that this functionality is not for initial chunks. (also documented in the architectural decisions of hf-inj-react)

This also introduces REACT_APP_INJECT_SCRIPTS which gets the entrypoints and makes them available in a given file.

[redbugz]

Normally env vars prefixed with REACT_APP are called that so they get forwarded to the client. I'm not seeing any usages of that env var in the client, so do we need it to start with REACT_APP?

[rileytb]

The entrypoints didn't have a full path (prepended w/the CDN when doing a prod build) So I needed to do this to know where to look for them. However, I am going to change this variable name because it's confusing with line 15. And it's just paths.publicUrlOrPathso it doesn't even need passed in

[rileytb]

Normally env vars prefixed with REACT_APP are called that so they get forwarded to the client. I'm not seeing any usages of that env var in the client, so do we need it to start with REACT_APP?

You're correct. They aren't used that way, I have changed both of those names.