[bugfix] Apply postformat to locale relative time methods - #6448

Summary

Apply locale postformat transformations to direct calls to:

• Locale#relativeTime
• Locale#pastFuture

Previously, postformat was only applied by higher-level APIs such as fromNow() and duration.humanize(). This caused direct locale method calls to return untransformed numbers for locales with symbol maps.

The internal relative-time helpers remain unformatted so composed humanized output is still postformatted only once.

Fixes #5935.

Tests

Added coverage for:

• Direct Locale#relativeTime postformatting
• Direct Locale#pastFuture postformatting
• The exact Nepali reproduction from the issue

Validation:

• pnpm test -- --only=moment/preparse_postformat
• pnpm test -- --only=locale/ne
• pnpm validate