feat: add receive and fallback functions with their respective tests

[KingFRANKHOOD]

An updated version of the CrowdFunding contract

• Create receive and fallback functions
• Test both functions to ascertain that they are working properly in addition to other tests
• Ensure all 34 tests are passing

• This is a CrowdFunding Contract that receives ETH contributions with a goal of 50 ETH and rewards it's contributors with an ERC20 token based on the size of their contributions
• The contract is thoroughly tested to ensure functional correctness of all it's logic

PR Checklist

• Tests
• Documentation