fix(intl): require explicit options next to a fallback in the Intl formatters

[dangreen]

Why

The overloads of number, datetime, relativetime, duration, list and range allowed a lone fallback, number(42), and the API page lists them as number(fallback?, options?). The runtime, however, treats the first argument as a fallback only when a second argument is present, so a lone fallback is passed to the Intl constructor as options and silently dropped:

number(42)(ctx)                   -> undefined
datetime(new Date(0))(ctx)        -> undefined
list(['red', 'green'])(ctx)       -> undefined
range(number, [1, 5])(ctx)        -> undefined
relativetime(-1)(ctx, 2)          -> RangeError: Invalid unit argument
duration({ hours: 1 })(ctx, {…})  -> RangeError: Value 1 out of range for options property hours

A runtime check by shape cannot fix this consistently: duration takes a plain object for both the fallback and the options, list accepts any iterable, datetime accepts a Date. A partial rule would cost bytes in the shared formatter and still leave some formatters needing explicit options.

What

• The fallback overloads of all six formatters require options ({} for the defaults, or false for raw mode). number(42) is a type error now, number(42, {}) and number(42, false) behave as before. No runtime code changes.
• The custom format overloads of number and datetime require the function and take the first two parameters as explicitly undefined-able, so they no longer swallow such calls. The internal RangeFrom type mirrors that shape.
• relativetime(options?) becomes relativetime(options): formatting without a unit threw a TypeError for any input.
• JSDoc of the fallback overloads mentions {} for the defaults, and the API page gets one sentence with the rule.

Checks

• oxlint, tsc --noEmit, vitest run (117 tests) in packages/intl pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.32%. Comparing base (1e29516) to head (d9b233a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #245   +/-   ##
=======================================
  Coverage   83.32%   83.32%           
=======================================
  Files          98       98           
  Lines        2554     2554           
  Branches      549      549           
=======================================
  Hits         2128     2128           
  Misses        314      314           
  Partials      112      112           

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