Fix incorrectly migrated package-lock.json

package-lock.json was migrated to "lockfileVersion": 2 by an old buggy
version of npm@7, and is therefore missing important information such
as the "bin" records that npm@7 needs for new installs.

    $ npm i; npm run build
    […]
    > [email protected] build
    > run-s build:pre build:build build:esm build:types build:post

    sh: line 1: run-s: command not found

Fix it by un-migrating with npm@6 and re-migrating with a current
npm@7.

npm/cli#2147 (comment)

Signed-off-by: Anders Kaseorg <[email protected]>

Author: andersk