Make highlight.js extension matching case-insensitive

What does this MR do and why?

The repository blob viewer ignored the highlight.js language overrides in FILE_EXTENSION_MAPPING_HLJS when the file name's casing differed from the map key. So UPPER.GLEAM and HELLO_UPPER.4DM showed as plain text, while lower.gleam was highlighted.

This MR lowercases both the file name and the extension before comparing them. That matches Rouge's own filename detection, which already ignores case.

References

• #631436 (closed)

Screenshots or screen recordings

File	Before	After
HELLO_UPPER.4DM
UPPER.GLEAM

How to set up and validate locally

1. In any project in your GDK, add these sample files (download all as a zip):

   • HELLO_UPPER.4DM
   • lower.4dproject
   • UPPER.GLEAM
   • MAIN.TF
   • hello.4dm and lower.gleam (controls that were already highlighted)

2. Open each file in the repository blob viewer.

3. Check that it's highlighted with the right language. The language is in the data-lang attribute on the .line elements:

   File	Before	After
   HELLO_UPPER.4DM	plaintext	4d
   lower.4dproject	plaintext	json
   UPPER.GLEAM	plaintext	gleam
   MAIN.TF	terraform (server-side Rouge fallback)	hcl
   hello.4dm, lower.gleam	4d, gleam	unchanged