Directory re-structure

[nkrishang]

Redo the directory structure of the repo:

contracts
|
|-- extension: "extensions that can be inherited by NON-upgradeable contracts"
|   |-- interface: "interfaces of all extension contracts"
|   |-- upgradeable: "extensions that can be inherited by upgradeable contracts"
|   |-- [$prebuilt-category]: "legacy extensions written specifically for a prebuilt contract"
|
|-- base: "NON-upgradeable base contracts to build on top of"
|   |-- interface: "interfaces for all base contracts"  
|   |--  upgradeable: "upgradeable base contracts to build on top of"
|
|-- prebuilt: "audited, ready-to-deploy thirdweb smart contracts"
|   |-- interface: "interfaces for all prebuilt contracts"
|   |--[$prebuilt-category]: "feature-based group of prebuilt contracts"
|   |-- unaudited: "yet-to-audit thirdweb smart contracts"
|       |-- [$prebuilt-category]: "feature-based group of prebuilt contracts"
|
|-- infra: "onchain infrastructure contracts used"
|   |-- interface: "interfaces for all infrastructure contracts"
|
|-- eip: "implementations of relevant EIP standards"
|   |-- interface "all interfaces of relevant EIP standards"
|
|-- lib: "Solidity libraries"
|
|-- external-deps: "modified / copied over external dependencies"
|   |-- openzeppelin: "modified / copied over openzeppelin dependencies"
|   |-- chainlink: "modified / copied over chainlink dependencies"
|
|-- legacy-contracts: "maintained legacy thirdweb contracts"