[v4] [icons] fix: use stable codepoints

[adidahiya]

Fixes #5150

Changes proposed in this pull request:

Add existing codepoints to icons.json using a one-time script like this:

// packages/icons/scripts/add-codepoints.js

const fs = require("fs");
const path = require("path");

const iconsMetadata = require("../icons.json");
const { IconCodepoints } = require("../lib/cjs/iconCodepoints");

for (const icon of iconsMetadata) {
    icon.codepoint = parseInt(IconCodepoints[icon.iconName], 10);
}

const newMetadata = JSON.stringify(iconsMetadata, undefined, 4);

fs.writeFileSync(path.resolve(__dirname, "../icons-new.json"), newMetadata);

And update scripts/generate-icon-fonts.js to pass along the codepoints to fantasticon so that they are stable when new icons are added.

[blueprint-bot]

fix formatting

Previews: documentation | landing | table | modern colors demo