runtime(sh): Update sh syntax and add shDerefOffset to shDerefVarArray for bash (vim/vim#13480)
Add shDerefOffset to shDerefVarArray.
Example code:
```bash
declare -a a=({a..z})
echo "${a[@]:1:3}"
```
ce3b0136c6
Co-authored-by: Lucien Grondin <grondilu@yahoo.fr>
d3e9cbedc7
