Implement system query for digit grouping rules

Since Qt 6, QLocale has included the CLDR data governing how to group digits correctly (where previously it grouped them in threes). However, the user may have configured rules at odds with the locale they are otherwise using (for example, es_ES requires at least two digits in the most significant group but some users prefer to allow a single-digit first group, as in many other locales). So we need a QSystemLocale::QueryType for the grouping rule, GroupingRule, for which we'll need to package three numbers into the return type; QLocaleData packs them into a single byte as two bits for the minimum size of most significant group, three bits each for the sizes of the least significant and all groups in between.

Given that CLDR has potentially separate grouping rules for currency digits, it'll probably make sense to add a

struct { int least, higher, top; }

that gets constructed out of the packed bits in QLocaleData or the return from QSystemLocale::query(); this implies an opportunity to possibly amend the bit-field to use some more tightly-packed form (e.g. all three fields are necessarily > 0, so storing the bit-field values with 1 subtracted from each would make two bits suffice for each, if they don't already).