feat: allow nested variable expansion in bash-like default value syntax

[iamfat]

add support to bash-like nested env syntax like B=${B:-BEGIN${A}END} and B=${B:=BEGIN${A}END}

[theskumar]

@iamfat The test for Typing seems to be failing. Can you please take a look?

[iamfat]

I fixed the typing problems. should be ok now.