Disassembly view: do not piggy back on existing step commands, introduce new ones

fyi @yuehuang010, @xisui-MSFT Code pointer https://github.com/microsoft/vscode/blob/main/src/vs/workbench/contrib/debug/browser/debugCommands.ts#L265 I already mentioned this in the PR review but it seems like we did not address this. We should not check the value of the context inside the command, but we should have special STEP commands for the disassembly view. That way the solution will be cleaner.