Add yarn fork command

[0xNeshi]

Closes #315

Explanation of the solution

Forks a Mumbai blockchain, where the Mumbai RPC URL is read from .env file.

@SovereignAndrey @stevieraykatz implemented it as-is for our immediate purposes where we're mostly debugging Mumbai, so set it as default. Can modify the script to include RPC URL of any blockchain so that we can fork both Polygon Mainnet, Goerli, Ethereum Mainnet etc., but this would require either:

• setting the RPC URL manually when calling the script
  OR
• creating a more complex script (possibly converting to bash script) that would accept network names as parameters, e.g. to fork a Mumbai blockchain we'd call yarn fork mumbai and to fork Ethereum Mainnet we'd call yarn fork ethereum etc.

Instructions on making this work

• run yarn or yarn install to install npm dependencies
• run yarn fork
• verify a forked Mumbai blockchain node is run locally