fix to export isBigNumber

[chaitanyapotti]

Description

Please include a summary of the change.
Added an export for isBigNumber so that it is available as

import { isBigNumber } from 'web3-utils'

Fixes the following issue

Type of change

• Bug fix
• New feature
• Breaking change
• Enhancement

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no warnings.
• I have updated or added types for all modules I've changed
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test in the root folder with success and extended the tests if necessary.
• I ran npm run build in the root folder and tested it in the browser and with node.
• I ran npm run dtslint in the root folder and tested that all my types are correct
• I have tested my code on an ethereum test network.

[coveralls]

Coverage increased (+0.001%) to 95.949% when pulling 81b5033 on chaitanyapotti:1.0 into dc63f75 on ethereum:1.0.

[coveralls]

Coverage increased (+0.001%) to 95.949% when pulling 81b5033 on chaitanyapotti:1.0 into dc63f75 on ethereum:1.0.