Enhance --changelog to support nested headings for changelogs following Keep a Changelog format

[mehanoid]

The --changelog flag is a great feature for updating CHANGELOG.md during version bumps. However, it currently does not handle nested headings, which makes it incompatible with popular changelog formats like Keep a Changelog.

For example, given the following changelog:

## [Unreleased]

### Added

- Very Useful Feature

## [0.0.1] - 2024-11-29

### Added

- Useful Feature

After running bump minor --changelog, the result is:

## [Unreleased]

### Added
## [0.1.0] - 2024-11-29

- Very Useful Feature

## [0.0.1] - 2024-11-29

### Added

- Useful Feature

The new version (0.1.0) is inserted under the nearest heading (### Added) instead of aligning with the heading level of the previous version (##). This disrupts the structure of changelogs that use nested headings for categories (e.g., ### Added, ### Fixed, etc.).

Expected behavior:

The new version should align with the same heading level as the previous version, resulting in:

## [Unreleased]

## [0.1.0] - 2024-11-29

### Added

- Very Useful Feature

## [0.0.1] - 2024-11-29

### Added

- Useful Feature

Proposed solution

Instead of inserting the new version under the closest heading, bump should search for the previous version heading at the same level (## in this example) and insert the new version after it. This would make the --changelog flag more compatible with nested heading formats.

[grosser]

sounds like a good plan, PR welcome 👍