keep redundant main field for webpack 4

This field is ignored by all modern tools when "exports" is present.

Author: dynst

package.json

@@ -2,6 +2,7 @@
   "name": "hash-wasm",
   "version": "4.12.0",
   "description": "Lightning fast hash functions for browsers and Node.js using hand-tuned WebAssembly binaries (MD4, MD5, SHA-1, SHA-2, SHA-3, Keccak, BLAKE2, BLAKE3, PBKDF2, Argon2, bcrypt, scrypt, Adler-32, CRC32, CRC32C, RIPEMD-160, HMAC, xxHash, SM3, Whirlpool)",
+  "main": "dist/index.cjs",
   "exports": {
     "require": "dist/index.cjs",
     "default": "dist/index.mjs"