fix: change main entry to CJS dist

[lukekarrys]

ESM aware tools will only look in exports (and not main) when resolving a package. Therefore the main entry point should always be a CJS file.

I believe this is just an oversight in the v1.x branch since the last commit touching package.json#main was 10 years ago.

This change will only affect older runtimes like node@<=10 as well as tools like resolve. I'm not sure if those Node versions are officially supported, but this change should be ignored by all new runtime versions and bundlers.

I found this due to a mocking library our tests were using that would fallback to the resolve package. In that case Node's require.resolve would end up pointing to a different path than resolve:

❯ node --version
v20.12.1
❯ node -e 'console.log(require.resolve("axios"))'
/Users/lukekarrys/Desktop/hmmmm/node_modules/axios/dist/node/axios.cjs
❯ node -e 'console.log(require("resolve").sync("axios"))'
/Users/lukekarrys/Desktop/hmmmm/node_modules/axios/index.js

Similar things happen in Node 10 that cause the require to throw:

❯ node --version
v10.24.1
❯ node -e 'console.log(require("axios/dist/node/axios.cjs").VERSION)'
1.6.8
❯ node -e 'console.log(require("axios").VERSION)'
/Users/lukekarrys/Desktop/wtf/node_modules/axios/index.js:1
import axios from './lib/axios.js';
       ^^^^^

SyntaxError: Unexpected identifier
    at Module._compile (internal/modules/cjs/loader.js:723:23)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Module.require (internal/modules/cjs/loader.js:692:17)
    at require (internal/modules/cjs/helpers.js:25:18)
    at [eval]:1:13
    at Script.runInThisContext (vm.js:122:20)
    at Object.runInThisContext (vm.js:329:38)