Different breakpoint marker for unverified breakpoint before debug session start

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> If a user sets a breakpoint before the debug session starts, it will look like a successful breakpoint in red. Then once the debug session starts, it might get registered and stay red or it might fail to register and turn grey. Unless the program stops on entry, gets paused, etc, the user might never even notice that the breakpoint failed to register and wonder why it never got hit. Displaying the preliminary breakpoint as registered is misleading. Users do not always realize that the validation will happen later and that they need to check that it succeeded ([example](https://github.com/golang/vscode-go/issues/1818#issuecomment-942745182), [example](https://github.com/golang/vscode-go/issues/1840)). I propose that a different marker is used for such breakpoints with a hover message that it will be validated once the debug session is launched.