Return only the key from interpolation_keys for sprintf-style patterns

[lenamonj]

I18n.interpolation_keys returned the sprintf format specifier alongside the key for the %<name>fmt form:

I18n.backend.store_translations(:en, greeting: "n = %<count>d, x = %<v>.2f")
I18n.interpolation_keys("greeting") #=> [["count", "d"], ["v", ".2f"]]

Regexp.union of the interpolation patterns has two capture groups for that pattern, and scan returns every group. interpolate_hash already picks the first non-nil capture as the key; interpolation_keys_from_translation now does the same, so the example returns ["count", "v"].

Four tests added. bundle exec rake passes on Ruby 3.3.8, 1611 runs (1607 before).