Add preprocessor support

Some kind of preprocessor support should be added to include unsafe/native code only in the output where it's really needed. This could be implemented similar to the C-preprocessor. E.g.

```golang
#if TYPESHELL_TARGET == "batch"
unsafe(`for %%U in ({0}) do (
	set "{o:1}=%%~zU"
	set "{o:2}=%%~tU"
	set "{o:3}=%%~aU"
)`, entry, sizeString, date, attributes)
#end
```