fix: main field in package.json should correspond to cjs artifacts

When axios#4787 was implemented, the project was switched to `"type": "module"` and "./index.js" became an esm file instead of commonjs, however, the "main" entry in package.json still points to "index.js". As a result, consumers using this field may get unexpected behavior since the main field is supposed to be commonjs if the entry is provided.

Many consumers won't run into this as a practical problem (for example when just doing `const axios = require('axios').default` from inside of a cjs file in node) because the "exports" map takes precedence over the main/module fields, but tools that don't parse the object map when resolving still run into problems here.

The fix for this is to just point the "main" entry-point to the commonjs artifacts located at "./dist/node/index.cjs".

package.json

@@ -2,7 +2,7 @@
   "name": "axios",
   "version": "1.4.0",
   "description": "Promise based HTTP client for the browser and node.js",
-  "main": "index.js",
+  "main": "./dist/node/axios.cjs",
   "exports": {
     ".": {
       "types": {
@@ -212,4 +212,4 @@
       "@commitlint/config-conventional"
     ]
   }
-}
+}