Validate Array Like Table Options

Options like `renderer.special_files` are validated only as a table; their contents are skipped as they are array-like.

Add functionality to validate the contents, in the above case strings.

Possibility:
```lua
local ACCEPTED_TYPES = {
  renderer = {
    special_files = "string[]",
  },
```

See #2655 : `filters.custom` cannot be validated as an array or a function.