Allow pinning react version (npm:styled-components uses a different version of npm:react)

[modeswitch]

I'm seeing an issue where npm:styled-components suddenly started using react@17.0.2, which obviously doesn't work when observable is pulling in the latest version of React (19.1.1 at the moment). I think this might be a problem with jsdelivr.

In order to address problems like this (by including a static version of styled-components, for example), it would be really helpful to be able to specify which React package we want observable to use during the build process. A config option to override the default behavior, something like

{
 ...
  jsx: "react@19.1.0"
 ...
}