feat: implement a setup function

Summary:

Now when using ivy.nvim you will need to call the `setup` function. This will
need to register any backends you want to use. This is an example config, this
can be put into a plugin in `~/.config/nvim/plugin/ivy.lua` for example.

```lua
require('ivy').setup {
  backends = {
    "ivy.backends.buffers",
    "ivy.backends.files",
  },
}
```

If you are using Lazy you can use the `config` directly to call the setup
function.

```lua
return {
  "AdeAttwood/ivy.nvim",
  build = "cargo build --release",
  config = {
    backends = {
      "ivy.backends.buffers",
      "ivy.backends.files",
    }
  }
}
```

The `setup` function can only be called once, if its called a second time any
backends or config will not be used. The module does expose the
`register_backend` function, this can be used to load backends before or after
the setup function is called.

```lua
require('ivy').register_backend("ivy.backends.files")
```

As well as the `register_backend` the core `run`function is exposed. With this
exposed we should be able to build anything we want.

```lua
vim.ivy.run(
  "Title",
  function(input)
    return {
      { content = "One" },
      { content = "Two" },
      { content = "Three" },
    }
  end,
  function(result) vim.cmd("edit " .. result) end
)
```

Test Plan:

Not much to test in this one, it has been tested locally on my config that does
not use any plugin managers, also a sandbox Lazy env using `NVIM_APPNAME`
NVIM_APPNAME

lua/ivy/init.lua

@@ -0,0 +1,28 @@
+local controller = require "ivy.controller"
+local register_backend = require "ivy.register_backend"
+
+-- Local variable to check if ivy has been setup, this is to prevent multiple
+-- setups of ivy
+local has_setup = false
+
+local ivy = {}
+ivy.run = controller.run
+ivy.register_backend = register_backend
+
+---@class IvySetupOptions
+---@field backends (IvyBackend | { ["1"]: string, ["2"]: IvyBackendOptions} | string)[]
+
+---@param config IvySetupOptions
+function ivy.setup(config)
+  if has_setup then
+    return
+  end
+
+  for _, backend in ipairs(config.backends) do
+    register_backend(backend)
+  end
+
+  has_setup = true
+end
+
+return ivy

plugin/ivy.lua

@@ -1,5 +1,4 @@
 local controller = require "ivy.controller"
-local register_backend = require "ivy.register_backend"
 
 -- Put the controller in to the vim global so we can access it in mappings
 -- better without requires. You can call controller commands like `vim.ivy.xxx`.
@@ -17,15 +16,4 @@ vim.paste = (function(overridden)
   end
 end)(vim.paste)
 
-register_backend "ivy.backends.buffers"
-register_backend "ivy.backends.files"
-register_backend "ivy.backends.lines"
-register_backend "ivy.backends.lsp-workspace-symbols"
-
-if vim.fn.executable "rg" then
-  register_backend "ivy.backends.rg"
-elseif vim.fn.executable "ag" then
-  register_backend "ivy.backends.ag"
-end
-
 vim.cmd "highlight IvyMatch cterm=bold gui=bold"