x/playground: allow escaping txtar delimiter

[x1unix]

Description

Let's imagine I want to play with x/tools/txtar package in a Go playground with a snippet like this:

package main

import (
        "fmt"
        "golang.org/x/tools/txtar"
)

// I want to print this string.
const str = `
-- a message --
`

func main() {
	fmt.Print(str)
        ar, err := txtar.Parse([]byte(str))
        if err != nil {
                panic(err)
        }
        
        // ....
}

Contents of a str are detected as a file delimiter as Go Playground uses txtar to bundle multiple files.
This leads to build error.

I thought maybe it's possible to escape -- character to bypass this issue but unfortunately Go Playground doesn't support that:

const str = `
\-- a message --
`

func main() {
	fmt.Print(str) // Prints "\-- a message --"
}

Proposal

Can you please provide some way to escape -- ... -- segments to workaround this issue or provide another better solution to this problem?

Thank you.

[seankhliao]

const str = `
`+`-- a message --
`

as an intentionally simple format, I think escape sequenced are out of scope for txtar.

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