chore: upgrade TypeScript to 3.9 (#5860)
chore: upgrade TypeScript to 3.9 (#5860)

Sticking to a specific version of TS rather than a ^ version so we know
everyone is on the exact same version. Think that's preferable for such
an important dependency.
GoogleChrome/puppeteer,puppeteer/puppeteer,GoogleChrome/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,GoogleChrome/puppeteer