Disable Go Companion when using a non-prerelease version of vscode-go

[firelizzard18]

Suppress Go Companion's duplicate features (test explorer, code lenses) when using a non-prerelease version of vscode-go to improve the developer experience as discussed on #3649. This should show the user a notification like the vscode-go experiment does so the user realizes that Go Companion is (mostly) disabled.

[gopherbot]

Change https://go.dev/cl/653115 mentions this issue: src/extensionAPI: expose isPreview

[firelizzard18]

This feature is ready but it won't work as expected until the next release (which will include 653115)