Simplify document type resolution

## Context

In the `contentlayer/source-files` plugin there are currently **two mechanisms** to tell Contentlayer how map a given document (e.g. a `.md` file) to one of many document definitions (e.g. a `BlogPost`, `Page`, ...). The two mechanisms are:

1. Explicitly via the `filePathPattern` option provided for `defineDocumentType`
2. Implicitly by looking up a "type" field in a given document (configurable via `fieldOptions.typeFieldName`)

## Problem

I'm seeing two problems with the current design:

#### A: Technical problems/limitations

When using mechanism (1) there can be situations where the provided `filePathPattern` can overlap between document definitions and there currently is no way to explicitly configure the "precedence" document matching behaviour of Contentlayer.

#### B: Hard to understand / learn

Additionally to the technical problems (A) having two different ways to do the same thing (especially when they overlap) makes things much harder to understand - especially for new Contentlayer users who e.g. might ask themselves "which approach should I use?" or "which approach is _better_?".

I think that providing less ways to do the same thing is the right approach in most cases as it reduces cognitive overload.

Additionally there's also the complexity resulting of the possibility of combining both mechanisms (1) + (2).

## Solution

As an alternative to the current mechanisms (1) + (2) I'm proposing a single unified approach to let users specify how to map document files to document type definitions by simply implementing a `resolveDocumentType` function as a property in `makeSource` like the following:

#### As replacement for mechanism (1)

Assuming a content file structure like:

```
.
├── about.md
├── index.md
└── posts
├── post-1.md
├── post-2.md
└── post-3.md
```

```ts
const Post = defineDocumentType(() => ({
name: 'Post',
fields: {
// ...
}
}))

const Page = defineDocumentType(() => ({
name: 'Page',
fields: {
// ...
}
}))

export default makeSource({
contentDirPath: 'posts',
documentTypes: [Post, Page],
resolveDocumentType: (frontmatter, raw) => raw.sourceFilePath.startsWith('posts/') ? 'Post' : 'Page'
})
```

#### As replacement for mechanism (2)

Assuming each document as a frontmatter field called `type`

```ts
const Post = defineDocumentType(() => ({
name: 'Post',
fields: {
// ...
}
}))

const Page = defineDocumentType(() => ({
name: 'Page',
fields: {
// ...
}
}))

export default makeSource({
contentDirPath: 'posts',
documentTypes: [Post, Page],
resolveDocumentType: (frontmatter, raw) => frontmatter.type
})
```

## Feedback wanted

We'd love to hear your feedback on the proposed API change. 🙏