fix: keep the last decimal separator when unformatting values

[DeoJin]

Summary

• make unformat() ignore extra decimal separator characters left behind by currency symbols like kr.
• preserve the final decimal separator so values such as kr. 123.45 parse as 123.45
• add a regression test covering the reported Swedish krona example

Validation

• node -e "const assert=require(''assert''); const accounting=require(''./accounting.js''); assert.strictEqual(accounting.unformat(''kr. 123.45'',''.''),123.45); assert.strictEqual(accounting.unformat(''$ 123,456.78''),123456.78); assert.strictEqual(accounting.unformat(''$ 123,456'','',''),123.456); console.log(''ok'');"

Fixes #240