feat: Improve frontmatter types, pass generic TFrontmatter type through

[BRKalow]

Update serialize() and compileMDX() to accept generics for scope and frontmatter. Pass through the generic to ensure that the return types reflect the provided frontmatter types.

serialize:

import { serialize } from 'next-mdx-remote/serialize'

interface Frontmatter {
  title: string
  published: string
  description?: string
}

//     👇 should have type Frontmatter
const { frontmatter } = serialize<Record<string, unknown>, Frontmatter>(source)

compileMDX:

import { compileMDX } from 'next-mdx-remote/rsc'

interface Frontmatter {
  title: string
  published: string
  description?: string
}

export default async function Page({ source }) {
	//              👇 should have type Frontmatter
	const { content, frontmatter } = await compileMDX<Frontmatter>(source)

	return (
		<>
			<h1>{frontmatter.title}</h1>
			{content}
		</>
	)
}

[thiskevinwang]

New typings look good to me! (via VSCode TS compiler)