fix: handle unformat symbols containing decimal separators

[DeoJin]

Summary

• add an optional symbol argument to unformat() so callers can strip currency symbols before decimal parsing
• fix the kr. case described in unformat() returns the wrong value if currency symbol contains the decimal separator #240 where the symbol contains the decimal separator
• add a regression spec covering both string and array inputs

Validation

• node -e "const accounting=require('./accounting'); function assert(cond,msg){ if(!cond){ console.error(msg); process.exit(1);} } assert(accounting.unformat('kr. 123.45', '.', 'kr.')===123.45,'regression failed'); const vals=accounting.unformat(['kr. 123.45','kr. 67.89'],'.','kr.'); assert(vals[0]===123.45 && vals[1]===67.89,'array regression failed'); assert(accounting.unformat('$ 123,456.78')===123456.78,'baseline failed'); console.log('ok');"