separate spacetime init/dev client and server templates

currently spacetime init/dev templates have the client and server together in the same template folder.
this is so that the client code is compatible with the server code but it also means that it is not possible to select different languages between the client and server. if all templates that contain the same code are named the same (except the language), then it wouldn't matter if the client and server templates are selected in two steps.

so to have for example a rust server and TS client, it would require to make a rust server only, and init another TS server + client and then delete the new TS server and replace it with the earlier created rust server.

it is also not possible to make just a client or just a server once the command is run. the spacetime init has a --server-only flag but it can only be used with the start of the command and not as option later. there is no flag for just a client.

there is also a flag for the server language but not for the client language, so it is not possible to use init without going through the dialogue if you want to have a client too.

Start from the spacetime init/dev command, its current template-selection flow, and the --server-only and server-language options described in the issue. Map how client and server templates are currently coupled; done means they can be selected independently, support different languages, and create client-only or server-only projects without the dialogue.