Improved handling of dependency updates

[niklasga]

Changesets can now have the type "dependency" and be grouped along with all other dependency updates in a separate list from any code changes.

A changeset with dependencies would then look like this:

---
"reactivewizard-parent": dependency
---

- io.projectreactor:reactor-bom: 2024.0.0

Rendered into a changelog it would be aggregated with other dependency updates under a separate heading like this:

### Dependency Updates

- io.projectreactor:reactor-bom: 2024.0.0

[Copilot]

Pull Request Overview

Adds support for treating dependency‐type changesets separately in changelogs and updates formatting/tests/docs to preserve multi­line bullets and render a new “Dependency Updates” section.

• Introduce a new DEPENDENCY level and update sorting, parsing, and rendering to group dependency changes.
• Enhance MarkdownFormatter to preserve trailing spaces for multi­line bullets.
• Update tests and documentation to expect and demonstrate the separate dependency section.

Reviewed Changes

Copilot reviewed 84 out of 85 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/main/java/se/fortnox/changesets/Level.java	Added DEPENDENCY enum and adjusted presentation strings.
src/main/java/se/fortnox/changesets/DependencyUpdatesParser.java	New parser for extracting unique dependency items.
src/main/java/se/fortnox/changesets/ChangesetParser.java	Map “dependency” to Level.DEPENDENCY in the switch.
src/main/java/se/fortnox/changesets/ChangelogAggregator.java	Inject and use DependencyUpdatesParser, adjust merge API.
src/main/java/se/fortnox/changesets/MarkdownFormatter.java	Adjust format flags to preserve trailing spaces in bullets.
src/test/java/se/fortnox/changesets/MarkdownFormatterTest.java	Added test for preserving multi­line bullet items.
src/test/java/se/fortnox/changesets/DependencyUpdatesParserTest.java	Tests for parsing unique dependency lines.
src/test/java/se/fortnox/changesets/CompareTest.java	Updated to expect new merge signature and dependency section.
src/test/java/se/fortnox/changesets/ChangelogAggregatorTest.java	New test shouldAggregateDependencyUpdates.
README.md	Document the new “dependency” changeset type and section.
.changeset/legal-parents-scream.md	Root changeset describing the new dependency feature.