Provide "suggestedFilename" in "WebDriver BiDi download started" hook. Addressing w3c/webdriver-bidi#840.

• At least two implementers are interested (and none opposed):
  • Chromium
  • Mozilla
• Tests are written and can be reviewed and commented upon at:
  • WebDriver BiDi browsingContext.downloadWillBegin WPT tests.
• Implementation bugs are filed:
  • WebDriver BiDi: Add reference to local filename to "browsingContext.downloadWillBegin" event w3c/webdriver-bidi#840
  • Chromium: feat: browsingContext.downloadWillBegin event GoogleChromeLabs/chromium-bidi#3248
  • Gecko: N/A
  • WebKit: N/A
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)