Supply @ffmpeg/core directly to createFFmpeg instead of as URL

**Is your feature request related to a problem? Please describe.**
I'm running ffmpeg.wasm in a browser environment but do not want to rely on loading a script from unpkg.com (it's annoying from a CORS perspective and goes against security and uptime best practices). Seemingly not a big deal since you can supply `corePath` but this is a little tricky with a bundler involved (particularly with Parcel which we use).

`ffmpeg-core.js` needs to be bundled since it does its own imports but it also needs to be reference by name and can't be in the same bundle. Code-splitting or dynamic importing would seem to be a solution but bundlers increasingly handle those transparently and don't expose the name.

**Describe the solution you'd like**
I'd like to be able to import `ffmpeg-core.js` myself and supply it to `createFFmpeg`, e.g.
```javascript
import FFmpegCore from '@ffmpeg/core/dist/ffmpeg-core'
const ffmpeg = createFFmpeg({ corePath: FFmpegCore });
```
or

```javascript
const FFmpegCore await import('@ffmpeg/core/dist/ffmpeg-core')
const ffmpeg = createFFmpeg({ corePath: FFmpegCore });
```

**Describe alternatives you've considered**
I've explored a few different bundlers and solving this problem on that level is quite annoying. You could also supply a version that bundles core but just allowing the user to supply it lets the whole thing be bundled together anyway.

**Additional context**
N/A