fix: preserve scalar keys ending in "[]" across stringify/parse

[spokodev]

ini.parse(ini.stringify(obj)) corrupts a scalar key that ends in []. stringify emits e.g. foo[]=bar, which is indistinguishable from the bracketedArray array marker, so parse strips the [] and wraps the value in an array — { 'foo[]': 'bar' } round-trips to { foo: ['bar'] }. Because bracketedArray defaults to on, this happens with the default options.

stringify now quotes a scalar key ending in [] (only in bracketedArray mode), and parse treats a quoted key literally rather than as an array marker. Real arrays and every existing case are unaffected — full suite green, 100% coverage.