fix: added support for generic package name

[Rechem]

The current version of this project doesn't support custom generic packages names, instead publishes them under the release package.

When publishing generic packages to the package repository on Gitlab, you are able to specify the name of your package in the URL : /projects/:id/packages/generic/:package_name/:package_version/:file_name

I added the ability to provide a custom package name through the optional packageName property.

Usage example:

{
  "branches": ["main"],
  "plugins": [
    "@semantic-release/commit-analyzer",
    "@semantic-release/release-notes-generator",
    [
      "@semantic-release/gitlab",
      {
        "gitlabUrl": "https://custom.gitlab.com",
        "assets": [
          { "path": "dist/asset.min.js", "label": "JS distribution", "target": "generic_package", "packageName": "styles" },
        ]
      }
    ]
  ]
}

This will create a new styles package in the Gitlab package repository with the specified files.

[Copilot]

Pull Request Overview

This PR adds support for custom generic package names in GitLab package publishing. Previously, all generic packages were published under the hardcoded "release" package name, but now users can specify a custom package name through an optional packageName property.

• Added packageName parameter support with "release" as default fallback
• Updated URL construction to use the custom package name in both upload and download endpoints
• Added comprehensive test coverage for the new functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/publish.js	Implements packageName parameter processing and updates generic package URL construction
test/publish.test.js	Adds test case verifying custom package name functionality with proper URL encoding