Allow cross-platform/per system initializeCommand definition

Not sure if it's the most proper repo to report this (if not, please redirect), but would be nice if there was a way to define `initializeCommand` per system in any way, so that the setup is truly cross-system. For example:
```
"initializeCommand": {
"windows": "rd /s /q somedir",
"linux": "rm -rf somedir",
"macos": "..."
}
```

Use case example: https://stackoverflow.com/questions/77199011/how-to-create-a-cross-platform-initializecommand-for-visual-studio-codes-dev-co (copying proper .env file before container launch).