hcldec: how to use `block_attrs`?

I don't understand how I'm supposed to use the `block_attrs` spec, and there are no examples in the hcldec documentation.

If I use the spec file here:
```
object {
block_attrs {
block_type = "locals"
element_type = any
required = false
}
}
```

I'm trying to parse a `locals` block filled with random attributes such as the hcl file here:
```
locals {
foo = "bar"
}
```

the latest version of hcldec outputs the following error:
```
Error: Unsupported block type

on spec.hcl line 2, in object:
2: block_attrs {

Blocks of type "block_attrs" are not expected here.
```

Start by reproducing the error with the provided spec.hcl and locals input, then read the hcldec documentation around block_attrs. Done means documenting valid block_attrs usage and including an example that parses a locals block containing arbitrary attributes.

Written by the indexing model from the issue text.