templates: release: Add JS to set default release title to tag name (#6645)

Closes #6485

Adds a bit of javascript in the template responsible for the "create release" page

When typing a name in the "tag name" field, the content will be automatically set in the "Title" field as a placeholder.

This way, you can type a version number (ex: `v5.0.2`), and the title will default to it (`v5.0.2` in this case)

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [x] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6645
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Litchi Pi <litchi.pi@proton.me>
Co-committed-by: Litchi Pi <litchi.pi@proton.me>

web_src/js/features/repo-release.js

@@ -43,6 +43,9 @@ function initTagNameEditor() {
       showElem('#tag-target-selector');
       tagHelper.textContent = value ? newTagHelperText : defaultTagHelperText;
     }
+
+    const title_input = document.getElementById('release-title');
+    title_input.placeholder = value;
   });
 }