Add syntax highlighting for .svelte files in the blob viewer

Summary

.svelte files are not syntax highlighted, and blank text is visible. svelte is one of major front end js frameworks. you can just alias to html, for basic support.

Workaround

If you are okay to use HTML highlighting in the meantime, you can force .svelte files to use HTML syntax highlighting by creating a .gitattributes file in your repository containing the following:

*.svelte gitlab-language=html