Add Chrome MV3 build target

Add a Chrome MV3 build target, that produces a main-world content
script ready for injection. Since there is not yet a way to pass
arguments to chrome.scripting.registerContentScript[1] registered
content scripts, generate a secret in the main-world (before the
website's own scripts have a chance to run) and emit that as an event
for our content script. That way, the main-world script still has a
way to share a secret with the content script for messaging.

Note: The registerContentScript does not yet allow about:blank frames
      to be targeted[2].

1 - https://developer.chrome.com/docs/extensions/reference/scripting/#method-registerContentScripts
2 - https://crbug.com/1360392