Run search in extension host as an experiment

[roblourens]

A sandboxed renderer process can't spawn the search process. We already run search in the EH in remote windows. We also still have a plan to enable extensions to contribute search providers which would run in the EH. We can add a setting to opt-in to running search in the EH all the time, and also enable it as an experiment to see if it causes any issues.

[roblourens]

(No verification needed, it's running this way in Insiders and won't in stable)

[bpasero]

@roblourens should this get a test plan item though?

[roblourens]

I guess verification-needed would be good.

• Check that text/file search features work
• Open the process explorer, there should be no "searchProcess"

[bpasero]

@roblourens is it expected that more than one rg process run though? On the vscode workspace with any exclusions disabled I searched and tried to monitor the running rg processes with htop: