feat: add Keypair class and deprecate Account (#17098)

* feat: add Keypair class and deprecate Account

* chore: fix lint issues

* chore: rename TransactionSigner to Signer

web3.js/src/index.ts

@@ -4,6 +4,7 @@ export * from './bpf-loader-deprecated';
 export * from './bpf-loader';
 export * from './connection';
 export * from './fee-calculator';
+export * from './keypair';
 export * from './loader';
 export * from './message';
 export * from './nonce-account';