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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See also:

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[github-actions]

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