allow web3-utils to work in projects with "noImplicitAny": true,

[joshstevens19]

Description

On TS projects if you set

"noImplicitAny": true,

TS will throw a error

ERROR in node_modules/web3-utils/types/index.d.ts(23,23): error TS7016: Could not find a declaration file for module 'bn.js'. 'C:/Users/Josh Stevens/repos/Project/node_modules/bn.js/lib/bn.js' implicitly has an 'any' type.
  Try `npm install @types/bn.js` if it exists or add a new declaration (.d.ts) file containing `declare module 'bn.js';`

We shouldn't force the developer to install that typing in their project so i have brought it in as a dependency in web3-utils which fixes this.

Type of change

• Bug fix
• New feature
• Breaking change

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 the live network.