`rg` will now be used over `ag` if it is available. This is because `rg`
is a faster alternative reduces lag when searching larger codebases.
`ag` is also now conditionally loaded if the command is available.
We now have a concept of a 'backend' this is the same as the current
sorters and finders but with added info like the keymap so they can all
be registered as one. This will allow us to split our backends into
modues so we can better maintain then.
The API for `window.set_items` took to many variable types. It would
take a table in multiple different formats and a string. Now it will
only take a table in a single format and a string. It will convert the
string into the table format by splitting it on new lines.
The table format is an array of tables that must have a `content` key
that will be the text that is displayed in the completion window. The
table can have any other data that is ignored.
```lua
local items = {
{ content = "Item one" },
{ content = "Item two" }
}
```
The `set_items` function will only display the `content` key in the
completion window, it will not do any sorting or filtering, that must be
done before passing the data to the `set_items` function.
