feat: Inactive group headers for `GroupMultiSelectPrompt`

[AdrianGonz97]

Closes #197

By default, group headers are selectable, allowing you to quickly select all of the options within that group. Setting the new option, selectableGroups, to false will disable them, making the group headers inactive.

the following code snippet:

import { groupMultiselect } from "@clack/prompts";

await groupMultiselect({
    message: "Which libraries would you like to setup?",
    selectableGroups: false,
    cursorAt: "Bootstrap",
    options: {
        CSS: [
            { label: "Bootstrap", value: "Bootstrap" },
            { label: "TailwindCSS", value: "TailwindCSS" },
            { label: "UnoCSS", value: "UnoCSS" },
            { label: "Bulma", value: "Bulma" },
        ],
        Testing: [
            { label: "Playwright", value: "Playwright" },
            { label: "Vitest", value: "Vitest" },
        ],
        Database: [
            { label: "Drizzle", value: "Drizzle" },
            { label: "Prisma", value: "Prisma" },
        ],
        Auth: [
            { label: "Lucia", value: "Lucia" },
            { label: "Auth.js", value: "Auth.js" },
        ],
    },
});

will now produce this prompt:

Vertical spacing can also be adjusted with a spacedGroups option as well:

[Mist3rBru]

Hey @AdrianGonz97, great work!

I just wanted to point out that the first group shouldn't have an empty line before it. In my opinion, this option should only add empty lines between the groups, not before or after. It looks a bit odd compared to the other prompts.