feat: support interceptors arrays

[antoinerey]

🔗 Linked issue

None.

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This commit allows passing interceptors as single functions (current behaviour), or arrays of functions (new behaviour). This makes it much easier to execute multiple functions on the consumer side. The alternative would be to compose all functions into one, but that's more complicated in my opinion.

With this change, the following becomes possible.

$fetch('/api/endpoint', {
  onRequest: [
    () => { /* Do something */ },
    () => { /* Do something else */ },
  ],
  onResponse: [
    () => { /* Do something */ },
    () => { /* Do something else */ },
  ],
})

Note that calling $fetch.create() and then passing interceptors when calling $fetch() will keep overriding the interceptors as the current behaviour. I did not want to go too much into there as I'm not sure what would be the expected behaviour. Plus, continuing to override the interceptors means that this PR is not a breaking change.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[antoinerey]

I'm not sure about this one. It seems simpler as interceptor types are only defined once, but the more verbose approach might also be viable.

onRequest?: 
  | ((context: FetchContext) => Promise<void> | void)
  | Array<(context: FetchContext) => Promise<void> | void>;

[antoinerey]

Friendly ping. 🤗 Is there anything I can do to make this PR move forward?

[enkot]

@antoinerey @pi0
I think it might be a better alternative to my PR (#357)
If we treat interceptors as an array, we can allow merging arrays from $fetch and $fetch.create. So that interceptors from $fetch and $fetch.create can be enforced to append or prepend to the resulting array of interceptors (similar to Nuxt plugins):

const $myFetch = $fetch.create({
  onRequest: [
    (ctx) => { /* Handler 1 */ }, // same as "enforce: 'default'"
    {
      enforce: 'post',
      handler: (ctx) => { /* Handler 2 */ }
    }
  ]
})

$myFetch({
  onRequest: [
    // Will be appended 
    (ctx) => { /* Handler 3 */ },
    // If you need to prepend in some scenarios
    {
      enforce: 'pre',
      handler: (ctx) => { /* Handler 4 */ }
    }
  ]
})

// Interceptors will be executed in this order:
/*
Handler 4
Handler 1
Handler 3
Handler 2
*/

Now I don't like my PR anymore 😅