extend pragma(msg) with writing to file

[a11e99z]

for example
for generating sources for other languages (interop)
or for viewing/checking auto generated code
```d
version (CS) pragma("interop.file.cs", ...);
pragma("just.to.check.generated.code.d", ...);
```

1) I know that exists security issues with it but IDK which one. 
so explain please.

2) allow writing file only to folder (compiler choice) **"g"** or 
**"gen"** for current source file or for current project.
disallow any changing folder symbols like '/', '\\', "..", and 
less than ASCII SPACE etc