Control the preference to always show the DevTools pages on startup from DevTools

VSCode has custom UX to let users opt into always running the inspector. The problem is it is surfaced before users have actually used the inspector so they aren't sure whether it is valuable. We should surface UX inside the embedded devtools to opt in and out of always showing the embedded devtools.

"You have used the inspector a lot, would you like to always launch it when you open DevTools?"

"You haven't interacted with the inspector in N days, would you like to stop seeing it when you debug a flutter app?"

DevTools server should be driving whether the inspector pages are automatically shown on startup from IDEs. That way DevTools can surface the option to opt in and out of always showing the embedded DevTools on startup.

Downside: weird for an IDE setting to be tracked in DevTools rather than the individual IDES. We could mitigate this if we wanted to get fancy by surfacing UX for this in the IntelliJ and VSCode plugin settings as well as DevTools but I'm not sure the complexity would be worth it.

Bonus points:
Also offer the option of always showing DevTools on startup for command line users. I'd personally like this option but I'm biased.

Start with the DevTools server behavior for embedded DevTools startup, then compare the VSCode and IntelliJ plugin settings with the command-line startup path. Define how DevTools should let users opt in or out based on inspector usage, and document whether IDE settings also need corresponding UX. Done means the startup preference works consistently across the requested entry points.

索引モデルが issue の本文から書いたものです。