This builds on devtools-html/cdp and devtools-html/tokio-cdp to implement a
simple Chrome DevTools Protocol server into Servo. The top-level browsing
context is exposed as the single "tab" supported (and thus creating new tabs and
switching between active tabs is not supported). One command is implemented so
far, Page.navigate, to navigate the tab to a particular URL.

Similar to the Firefox DevTools Server, the Chrome DevTools Server can be
started by passing the --cdp flag to Servo. The default port is 9222; a
different one can be specified by passing a port to the flag.