feat: Implement ParseError and IsParseError

[LumaKernel]

resolves #101

Hello! It's my first time PR for golang project :D Please let me know if there're unfamiliar parts. Thanks.

Usage example:

var err error
err = godotenv.Load(".env")
if godotenv.IsParseError(err) {
  log.Fatal(fmt.Errorf("%s: %s", ".env", err))
}
err = godotenv.Load(".env.develop")
if godotenv.IsParseError(err) {
  log.Fatal(fmt.Errorf("%s: %s", ".env.develop", err))
}