runtime(sh): add shDblParen to shLoopList for bash (vim/vim#13445)
add shDblParen to shLoopList to correctly highlight
arithmetic expressions for Bash and Ksh
This should allow code such as:
```bash
declare -i i j
for i in foo bar
do ((j = 1 << j))
done
```
a390e984db
Co-authored-by: Lucien Grondin <grondilu@yahoo.fr>
a5629abce2
