Compile only the needed search JS modules

When the frontend is compiled, some items that take the longest to compile are the various searches.

We should skip compiling them if BACKEND_SEARCH='' is defined. Or, compile only those defined by the variable.

I see in https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/rollup.config.js we can maybe set const SEARCH_BACKEND = process.env.SEARCH_BACKEND || 'algolia'; and then use that to selectively compile the searches.