GH-45250: [JS] Fix denominator precision loss and remove unnecessary safe integer check for fractional part

[yaooqinn]

Rationale for this change

What changes are included in this PR?

This PR uses string constructor for bigint to calculate the power of 10 with scale as the exponent value of the expression.

To Fix precision loss like

> BigInt(10) ** BigInt(25);
10000000000000000000000000n
> BigInt(Math.pow(10, 25))
10000000000000000905969664n

Also, we remove the unnecessary safe integer check for the fraction part.

Are these changes tested?

add some unit tests

Are there any user-facing changes?

no

• GitHub Issue: bigNumToNumber method may result precision loss for denominator calculation #45250

[github-actions]

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[github-actions]

⚠️ GitHub issue #45250 has been automatically assigned in GitHub to PR creator.

[trxcllnt]

Minor suggestion, but we should be able to use the String built-in and remove the padStart below this:

Suggested change

	const fractionPart = padStart((remainder).toString(), scale);
	const fractionPart = `${remainder}`.padStart(scale, '0');

[trxcllnt]

Suggested change

	function padStart(str: string, targetLength: number) {
	while (str.length < targetLength) {
	str = '0' + str;
	}
	return str;
	}

[trxcllnt]

Can use padEnd() here:

Suggested change

	const denominator = BigInt('1' + '0'.repeat(scale));
	const denominator = BigInt('1'.padEnd(scale + 1, '0'));

[trxcllnt]

Suggested change

	const sign: string = negative && integerPart === 0 ? '-' : '';
	const sign = negative && integerPart === 0 ? '-' : '';

[trxcllnt]

IIRC coercing via +str is a bit faster than using the Number constructor. Minifiers will typically do this as well:

Suggested change

	return Number(`${sign}${integerPart}.${fractionPart}`);
	return +`${sign}${integerPart}.${fractionPart}`;