Add infinite scrolling to alphabetical index

[UnniKohonen]

Reasons for creating this PR

Alphabetical index does not load more concepts as it's scrolled. This PR adds functionality to address this.

Link to relevant issue(s), if any

• Part of Implement alphabetical view in sidebar using Vue #1563

Description of the changes in this PR

• Add an event listener for scrolling alphabetical list
• Detect when scrolling reaches the bottom of the list and load more concepts
• Add messages for loading content to SKOSMOS variable
• Add spinners when loading content

Addresses requirement 2 in #1563

Known problems or uncertainties in this PR

• There could still be some bugs with loading (i.e. loading happening in the wrong order, concepts being loaded twice, some offsets being skipped, etc).

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[joelit]

This could be a value from the window.SKOSMOS object, e.g. "Ladataan sisältöä", "Laddar mera resultat" & "Loading more items". Also if you want to use a spinner, something like <i class="fa-solid fa-spinner fa-spin-pulse"></i> could be considered.

[osma]

Not sure if it's worth adding more translations to the SKOSMOS object, because the translation mechanism for the frontend may still change to something completely different that doesn't rely on the SKOSMOS object (e.g. reading the translations directly from a JSON file). See #1525 and #1523 (comment) for some thoughts

[joelit]

Same here as on L183

[joelit]

It works well now! A minor thing, but I'd get rid of the hardcoded loading message at this point already.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[joelit]

LGTM!