errors: add errors.Mark to support error marks

[searKing]

errors: add errors.Mark to support error marks for errors.Is and '%v' in fmt, without any side effect otherwise
playgroud for error with marks

package main

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
)

func main() {
	var markErr = errors.New("mark")
	_, err := os.Open("non-existing")

	err = errors.Mark(err, markErr)

	if err != nil {
		if errors.Is(err, fs.ErrNotExist) {
			fmt.Println("file does not exist")
		}
		if errors.Is(err, markErr) {
			fmt.Println("a mark err occur: ", err)
		}

		if !errors.Is(err, fs.ErrNotExist) && !errors.Is(err, markErr) {
			fmt.Println(err)
		}
	}

	// Output:
	// file does not exist
	// a mark err occur:  open non-existing: no such file or directory
}

[gopherbot]

This PR (HEAD: e8e21bc) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/go/+/380694 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://golang.org/doc/contribute.html#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11 or adds a tag like "wait-release", it means that this CL will be
reviewed as part of the next development cycle. See https://golang.org/s/release
for more details.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/380694.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1: Code-Review-2

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/380694.
After addressing review feedback, remember to publish your drafts!

[searKing]

This is a PR / CL / implementation Linked to #50819