Adds two internal per-server options: `packages.prefix` & `packages.suffix`.
These options allow the server module to install multiple packages, and
control which end up being prefixed or suffixed on the PATH.
This simplifies the propagating code in `modules/lsp/servers/default.nix`,
which can now zip up the enabled server `packages` attrs.
Adding a module in the following places will import the module into that
specific server's submodule.
This allows creating server-specific options.
Added a test case to ensure all custom modules correspond to an actual
server option to avoid accidental dead code.
