Fix 19387 - Mark generated postblits as scope

[MoonlightSentinel]

Otherwise the postblit is not callable when compiling with DIP 1000.
(Assigning scope ... to non-scope this)

[RazvanN7]

I think you need to include the word "Issue" in the commit message so that the bot picks it up.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19387	normal	[dip1000] __fieldPostblit should be scope or deduce scope qualifier

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 + dmd#12280"

[Geod24]

I think you need to include the word "Issue" in the commit message so that the bot picks it up.

Nope, we changed that a year ago to allow for shorter title.