plaintext does not render empty lines (\n line breaks)

[ben519]

When I set the language to plaintext, it loses formatting and copy-paste'ability regarding empty lines. For example,

```plaintext
Hello

World

becomes

Hello
World

(Same thing happens when I set `defaultLang: "plaintext"`)

---

@atomiks I'd like to sponsor you if you're open to it. Appreciate the hard work you put into this package!

[atomiks]

Yeah this is due to the display: grid option.

That's what this part fixes, which you saw in the other issue:

onVisitLine(node: { children: string | any[] }) {
  // Prevent lines from collapsing in `display: grid` mode, and allow empty
  // lines to be copy/pasted
  if (node.children.length === 0) {
    node.children = [{ type: "text", value: " " }]
  }
},

This is done internally for the grid: true option, but I think it's skipped for plaintext language, since it returns early to avoid highlighting...

---

btw, thank you for the offer, but I don't have a GitHub Sponsor page set up

[ben519]

Thanks for the quick patch! If you set up a sponsor page, let me know.