Fix: Replace ambiguous headers types with actual types

[ryami333]

Currently the headers types (both in the context of request/config and response) are ambiguously (any) typed, which prevents Typescript from being able to guard against obvious mis-configuration or mis-use of the response data.

Pull request type

Please check the type of change your PR introduces:-->

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Other (please describe):

How to test this PR

1. Create an instance of the Storyblok client, and try to configure both string-valued headers, and non-string-valued headers. You should expect a Typescript error when you pass a non-string value (and no error otherwise).
2. Call a getter method like getStory and inspect the type of response.headers. It should be of type Headers (with .get methods, et al).

const client = new Storyblok({
  headers: {
    'Content-Type': 'application/json',
    'X-Foo': [1, 2, 3], // Error: Type 'number[]' is not assignable to type 'string'
  },
})

client.getStory('xxxxxxx', {}).then((response) => {
  const contentType = response.headers.get('Content-Type') // Type: string | null
})

What is the new behavior?

No new functional behaviour - only changes to types to reflect actual runtime usage.

Other information

None.

[ryami333]

Lots of whitespace/indentation changes because this file had not been correctly formatted with Prettier. See this PR where I set a formatting baseline. Once that is merged I can rebase this PR to make the diffs less noisy.

In the meantime, please hide whitespace changes in code-review.

EDIT: rebased onto main, no more formatting changes.