feat(ui): improve multiline file preview and anchor detection (#9145)

This PR solves some little annoyance for me by allowing line ranges for inline file previews and source links to be of the form `L1-9` instead of necessarily `L1-L9`.
For links to source files it allows also `n1-9` or `n1-n9` in agreement with already allowed single line anchors `n1`.

### Tests

- I added test coverage for Go changes...
  - [x] 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...
  - [x] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] 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

- [ ] I do not want this change to show in the release notes.
- [x] 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/9145
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Robert Wolff <mahlzahn@posteo.de>
Co-committed-by: Robert Wolff <mahlzahn@posteo.de>

web_src/js/features/repo-code.test.js

@@ -14,4 +14,7 @@ test('rangeAnchorRegex', () => {
   expect(rangeAnchorRegex.test('#L1-L10')).toEqual(true);
   expect(rangeAnchorRegex.test('#L01-L10')).toEqual(false);
   expect(rangeAnchorRegex.test('#L1-L01')).toEqual(false);
+  expect(rangeAnchorRegex.test('#L1-10')).toEqual(true);
+  expect(rangeAnchorRegex.test('#n1-n10')).toEqual(true);
+  expect(rangeAnchorRegex.test('#n1-10')).toEqual(true);
 });