fix(relativeTime): use 1 for singular thresholds instead of the previous unit's value (#2956)

[anas510]

Fixes #2956, fixes #2695

Singular thresholds (m, h, d, M, y) have no diff unit, so fromNow() / humanize() reused the count computed for the previous unit. Locales or updateLocale configs that use %d in singular keys printed e.g. 28 month ago, 60 hour ago, or 30 month for a 1-month duration.

• Thresholds without a diff unit now substitute 1, matching Moment (number || 1).
• Default en output and all shipped locales are unchanged (no singular key uses %d, and function locales ignore the number for singular keys).
• Added tests for fromNow/toNow and duration().humanize() with %d singular keys.