feat: trigger RocksInstallPost user event after install/update

[mrcjkb]

See also: #489

This would allow users of plugins that require an impure autoload function to be invoked to do so.
For example:

vim.api.nvim_create_autocmd("User", {
   pattern = "RocksInstallPost",
   callback = function(ev)
      if ev.data.installed.name == "firenvim" then
        vim.cmd.call("firenvim#install(0)")
      end
  end,
})