Fix partially Issue 21573 - formatting floating points should be CTFE

[berni44]

I just noticed, that CTFE can already be implemented independent of #7757 and followups. Of course, it will be limited to the part, where snprintf is already replaced, currently %e and %a applied to float and double.

Thanks to Petar Kirov and Paul Backus for help.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21573	enhancement	formatting floating points should be CTFE

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7762"

[atilaneves]

I guess this can't be pure because of the floats?

[berni44]

Surprisingly (for my) it can be pure, while the quite similar test directly above cannot. The static seems to make a difference here.

[berni44]

removed merge conflict with #7764