Path.expandVars

Expands environment variables in the path string.

Supported patterns:

• POSIX (all platforms): $VAR and ${VAR} expand to the value of VAR.

If the variable is not defined, the reference is left unchanged.

• Windows (in addition to $VAR/${VAR}): also expands %VAR%.

Notes:

• Empty variables expand to an empty string.
• Braced form ${VAR} takes precedence when both styles are present.
• This function does not interpret quotes or escapes; it only performs textual expansion.