Update caniuse-lite in build process

Problem/Motivation

Theme build process complains often with next message:

Browserslist: caniuse-lite is outdated. Please run:
  npx browserslist@latest --update-db
  Why you should do it regularly: https://github.com/browserslist/browserslist#browsers-data-updating

And then you need to manually execute `npx browserslist@latest --update-db` for all custom themes.

This is a bit boring to do all the time.

Proposed resolution

Add `npx browserslist@latest --update-db` command into package.json `build-css` so it is executed every time, so it should look like this:

  "scripts": {
    "build-css": "((type pnpm && pnpm install) || npm install) && npx browserslist@latest --update-db && npx gulp clean:css && npx gulp sass"
  }

Also add update function that is updating all child themes package.json with this.