chore: use `files` field, instead of .npmignore

[ghost]

Description

This PR uses files field in package.json instead of using .npmignore. It's easier to manage allowlist (files field in package.json), compared to blocklist (.npmignore).

Also, it prevents .prettierrc and .gitmodules being published. In operator-filter-registry v1.3.0, the unnecessary .prettierrc and .gitmodules are included.

For testing, you can run npm publish --dry-run which shows the following:

npm notice 📦  operator-filter-registry@1.3.0
npm notice === Tarball Contents === 
npm notice 1.1kB  LICENSE                                                        
npm notice 11.3kB README.md                                                      
npm notice 487B   package.json                                                   
npm notice 497B   src/DefaultOperatorFilterer.sol                                
npm notice 1.8kB  src/example/ExampleERC721.sol                                  
npm notice 1.6kB  src/example/ExampleERC1155.sol                                 
npm notice 2.2kB  src/example/RevokableExampleERC721.sol                         
npm notice 2.0kB  src/example/RevokableExampleERC1155.sol                        
npm notice 2.0kB  src/example/upgradeable/ExampleERC721Upgradeable.sol           
npm notice 1.8kB  src/example/upgradeable/ExampleERC1155Upgradeable.sol          
npm notice 2.4kB  src/example/upgradeable/RevokableExampleERC721Upgradeable.sol  
npm notice 2.2kB  src/example/upgradeable/RevokableExampleERC1155Upgradeable.sol 
npm notice 2.1kB  src/IOperatorFilterRegistry.sol                                
npm notice 2.7kB  src/OperatorFilterer.sol                                       
npm notice 22.0kB src/OperatorFilterRegistry.sol                                 
npm notice 1.5kB  src/OperatorFilterRegistryErrorsAndEvents.sol                  
npm notice 697B   src/OwnedRegistrant.sol                                        
npm notice 781B   src/RevokableDefaultOperatorFilterer.sol                       
npm notice 3.0kB  src/RevokableOperatorFilterer.sol                              
npm notice 3.8kB  src/UpdatableOperatorFilterer.sol                              
npm notice 493B   src/upgradeable/DefaultOperatorFiltererUpgradeable.sol         
npm notice 2.7kB  src/upgradeable/OperatorFiltererUpgradeable.sol                
npm notice 556B   src/upgradeable/RevokableDefaultOperatorFiltererUpgradeable.sol
npm notice 3.0kB  src/upgradeable/RevokableOperatorFiltererUpgradeable.sol       
npm notice === Tarball Details === 
npm notice name:          operator-filter-registry                
npm notice version:       1.3.0                                   
npm notice filename:      operator-filter-registry-1.3.0.tgz      
npm notice package size:  12.9 kB                                 
npm notice unpacked size: 72.9 kB                                 
npm notice shasum:        c435321b830147490ccb01fa477eda2be10fb836
npm notice integrity:     sha512-MQMyiefDpmDzM[...]m7LVYHHYvsGjg==
npm notice total files:   24                                      
npm notice 
npm WARN This command requires you to be logged in to https://registry.npmjs.org/ (dry-run)
npm notice Publishing to https://registry.npmjs.org/ (dry-run)

[operatorfilterer]

Thanks for this!