Fix sys.debugMode when using VSCode's new preview debugger

[rbuckton]

VSCode's new "preview" debug extension doesn't use --inspect (or --inspect-brk) to launch the debugger, so this adds a new heuristic to detecting whether the compiler was launched from the debugger.

The preview debugger launches the program with something like the following script (this example uses the powershell console):

${env:NODE_ENV}='development';
${env:NODE_INSPECTOR_IPC}='\\.\pipe\node-cdp.15176-2.sock';
${env:NODE_INSPECTOR_PPID}='';
${env:NODE_INSPECTOR_WAIT_FOR_DEBUGGER}='';
${env:NODE_OPTIONS}='--require C:\Users\<user>\AppData\Local\Temp\vscode-js-debug-bootloader.js ';
${env:NODE_INSPECTOR_EXEC_PATH}='C:\Users\<user>\AppData\Roaming\nodejs\node.exe';
${env:VSCODE_DEBUGGER_FILE_CALLBACK}='C:\Users\<user>\AppData\Local\Temp\node-debug-callback-<id>';
${env:VSCODE_DEBUGGER_ONLY_ENTRYPOINT}='false';
& 'C:\Users\<user>\AppData\Roaming\nodejs\node.exe' 'C:\dev\TypeScript-bugfix/built/local/tsc.js'

The new heuristic checks for the presence of the NODE_INSPECTOR_IPC environment variable in addition to checking for --inspect or --debug.