[feature request] "changeset" support

[Hebilicious]

Describe the feature

Motivation

It would be great if changelogen had the ability to support an optional "changeset" feature. This would allow automated changelog and release logs to include more detailed information about changes. This information can be written for each PR using a markdown file.

Details :

Here's how this could work :

1. When making a PR, add a .changelogen/[generated-name].md using the changelogen cli.
2. In this file, write a markdown description of the change.
3. When a release is made, the changelogen cli add these changes into each package CHANGELOG.md, and delete the "changesets".
4. The release text is pre-populated with each "changeset" and can be manually modified.

Proposed Format :

Changeset

---
'@nuxt/kit': PRID / commitHash
---

Here we can write a more *detailed* change, using markdown formatting.
| 1 | 2 | 3 |
|---|---|---|
| We | can use | tables |  

Changelog with changeset

🚀 Enhancements

- First Change (https://github.com/unjs/changelogen/pull/123)

- Second Change (https://github.com/unjs/changelogen/pull/123)

	Here we can write a more *detailed* change, using markdown formatting.
	| 1 | 2 | 3 |
	|---|---|---|
	| We | can use | tables |  

- Third Change (https://github.com/unjs/changelogen/pull/123))

Related issues

#85 For monorepo support this is able to correctly move the changeset text into each package changelog.
#91 This must be connected to pre-releases
#104 This must be integrated with a github app/action

Additional information

• Would you be willing to help implement this feature?

### Tasks
- [ ] https://github.com/unjs/changelogen/issues/85
- [ ] https://github.com/unjs/changelogen/issues/104
- [ ] https://github.com/unjs/changelogen/issues/91