Add CreateFormatterObject AO

[jessealama]

Adds the CreateFormatterObject abstract operation, which builds the formatter object (with internal slots for rounding mode, digit limits, etc.) needed by FormatNumericToString. Replaces the TODO placeholders in the Amount constructor and convertTo with actual calls to this new AO. Spun off from #89 per reviewer feedback. Closes #93.

[github-actions]

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[gibson042]

This is a change for the better, but is not currently addressing #89 (comment) .

[gibson042]

This looks good after fixing the typing to correctly map integral Numbers to integers where required.

And for a potential followup, there's a common pattern here that has me wondering whether a new operation is warranted:

  1. If _x_ is not *undefined*, set _x_ to ? SnapToInteger(_x_, ~strict~, 1, 21).

where the second parameter is either ~strict~ to reject non-integral output from ToNumber or ~truncate-strict~ to reject non-finite output but truncate finite non-integers, and the optional third and fourth parameters respectively define inclusive lower and upper bounds.

Such an operation could replace Temporal ToIntegerIfIntegral(x) with SnapToInteger(x, ~strict~), Temporal ToIntegerWithTruncation(x) with SnapToInteger(x, ~truncate-strict~), and Temporal ToPositiveIntegerWithTruncation(x) with SnapToInteger(x, ~truncate-strict~, 1). It could potentially even be extended to consolidate almost all integer conversions by introducing a ~truncate-loose~ behavior (truncating finite non-integer output from ToNumber and mapping NaN to 0) for use in the definition of ToLength and ToIndex (respectively, 𝔽(the result of clamping SnapToInteger(argument, ~truncate-loose~) between 0 and 2⁵³ - 1) and SnapToInteger(argument, ~truncate-loose~, 0, 2⁵³ - 1)):

Operation	NaN	±∞	Finite non-integer	Bounds	Behavior
ToIntegerOrInfinity	→ 0	preserve	truncate		n/a
ToIntegerIfIntegral (Temporal)	RangeError	RangeError	RangeError		STRICT
ToIntegerWithTruncation (Temporal)	RangeError	RangeError	truncate		TRUNCATE-STRICT
ToPositiveIntegerWithTruncation (Temporal)	RangeError	RangeError	truncate	RangeError if ≤ 0	TRUNCATE-STRICT
ToLength	→ 0	preserve	truncate	clamp to [0, 2**53 - 1]	TRUNCATE-LOOSE
ToIndex	→ 0	preserve	truncate	RangeError if outside [0, 2**53 - 1]	TRUNCATE-LOOSE
DefaultNumberOption (ECMA-402)	RangeError	RangeError	preserve	RangeError if outside [min, max]1	n/a1
GetAmountOptions	RangeError	RangeError	RangeError	RangeError if outside [min, max]	STRICT
GetAmountConvertToOptions	RangeError	RangeError	RangeError	RangeError if outside [min, max]	STRICT

Footnotes

1. DefaultNumberOption currently checks output from ToNumber against specified bounds before truncation: https://github.com/tc39/ecma402/issues/691 ↩ ↩²

[gibson042]

LGTM, and note the below improvement suggestions.