backslashes in JS string are lost in zig

When a zig funtion gets a js string as argument, `\` characters are lost.

We can reproduce by adding a following js test:
```zig
.{ "'foo\\:bar'", "foo\\:bar" },
```

the result is:
```Expected:
foo\:bar

Got:
foo:bar

Case: 10
'foo\:bar
```