extend pragma(msg) with writing to file

[Nicholas Wilson]

On Thursday, 6 October 2022 at 09:55:34 UTC, a11e99z wrote:
> for example
> for generating sources for other languages (interop)

are there other languages you would like to be supported?
C/C++ are available with `-HC` family of switches.

> or for viewing/checking auto generated code

mixin has this option already:
`-mixin=<filename>` expand and save mixins to file specified by 
`<filename>`