Implement an error-comparing method to errorsx String

[sryoya]

PR preparation

What

• implemented an error-comparing method Is() to the struct String in the package errorsx
• create a test file for errorsx on the way to make a test for this func

Why
To make it possible to compare the custom error (String in errorsx) with all the struct implementing error interface by errors.Is.
That enables users of this package to check returned exported errors in their own applications by the latest Go standard error comparing way.

For example, the package user can handle the returned error like this

if errors.Is(err, slack.ErrRTMDisconnected) {
  // do something
}

From Go1.13, errors.Is is provided as a function of a Go standard package.
And, to make the custom error comparable by this func, we need to implement Is method to that.
From the package description of errors

An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.

An error type might provide an Is method so it can be treated as equivalent to an existing error. For example, if MyError defines

func (m MyError) Is(target error) bool { return target == os.ErrExist }

then Is(MyError{}, os.ErrExist) returns true. See syscall.Errno.Is for an example in the standard library.

Note
Firstly I meant and tried to update result evaluation in some test files by this func, but it was too early considering the consistency for the old Go version. So, now I propose this func just for increasing the package user's options to check the returned error from this package.

[kanata2]

👍
Could you put commits together?

Note: I'm currently considering only supporting Go 1.13+.

[sryoya]

Could you put commits together?

done 👍

Note: I'm currently considering only supporting Go 1.13+.

I agree with that. I think enough time has passed since Go1.13 was released.

[kanata2]

Looks good to me :)