feat: export mergeConfig

[kevincennis]

Export the mergeConfig utility function, as has been previously requested here.

I believe tacking mergeConfig directly on to the default export is the simplest and most flexible solution, since adding lib/core/mergeConfig.js to the exports of package.json would require additional work on the build step to support CJS – and it also seems like a bit of an anti-pattern to expose individual files like that.

I'm happy to discuss use-cases if anyone needs more convincing, but I think the author of #3905 already did a great job with that. The TL;DR is basically "the ability to merge configs within an interceptor is very useful, and trying to re-create that functionality is error-prone because mergeConfig has a ton of logic that's very specific to axios internals".

[jasonsaayman]

@kevincennis please check the build it seems to be failing.

[kevincennis]

Yeah. Some new tests got added after I created the PR. One sec.

[jie-huang]

Can axios export mergeConfig in index.d.ts file too?

So typescript can use it as

import { mergeConfig } from 'axios';