[file_selector] Annotate all creation of XTypeGroup with // ignore: prefer_const_constructor

[eugerossetto]

Annotate all creation of XTypeGroup with // ignore: prefer_const_constructor to prepare this implementation for the const constructor of XTypeGroup.

#111906 [file_selector] XTypeGroup should be immutable

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[stuartmorgan-g]

See #6463 for comments that apply here as well (including the fact that we should ideally combine the PRs).

[eugerossetto]

This will be added to #6463.