* Implement __ctfeWrite builtin
Reworked stalled PR #12412.
The __ctfeWrite function is already part of druntime (core.builtins), but was never implemented.
The actual implementation is the one used at Weka (slightly different from PR #12412) and does not require any changes to expression.d (contrary to said PR).
* fix file extension of changelog entry
* fix changelog file naming
* Improve changelog entry
* Remove superfluous code and add test case.
