Fix Issue 21846 - std.format: provided format string for toString does not work with grouping

[berni44]

The separator needs to be after width. I don't know though, if it should be before or after precision. The current implementation allows both.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21846	normal	std.format: provided format string for toString does not work with grouping

Testing this PR locally

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dub run digger -- build "master + phobos#7996"