error/compat issue: variable substitution broken

so i ran the command `echo "${foo-'}'}"` on regular mode [non-posix] and, much like bash, it echoed `'}'`. this is not the issue. the issue is that when run in posix mode, the same is outputted:
```
🍣 echo "${foo-'}'}"
'}'
```
can you guys fix this ? this seems like a really nice project, especially to go with the uutils suite of tools :3