Prevent multiline image alt text

[JTS-FIN]

Many (most?) markdown parsers don't support multiline img alt-texts (Hugo for example). So this is a fix for that.

Replacing from html img multiline alt-texts to single line. Replacing newlines with " - ". Multiple newlines will be replaced by only one " - ".

[JTS-FIN]

Here is also how I tested it:

// test-multiline-img-alt-text.js
const TurndownService = require('./lib/turndown.cjs.js')

const turndownService = new TurndownService()
const markdown = turndownService.turndown(`<img src="example.jpg" alt="Example Image
with

multiline alt text" />`)
console.log(markdown)

Resulted in:

![Example Image - with - multiline alt text](example.jpg)