Update cuda source file types

[NellyWhads]

Replicates the file type structure from C files to Cuda files.

• Source file convention remains *.cu
• Header file convention is added to include *.cuh files

The community-adopted *.cuh convention is exemplified in many large-scale public repositories, including torch.

[trunk-io]

⏱️ 1h 11m total CI duration on this PR

Job	Cumulative Duration	Recent Runs
Linter Tests ubuntu-latest	42m	🟩 🟩
Linter Tests macOS	23m	⬜ 🟩
CodeQL-Build	3m	🟩 🟩
Trunk Check runner [linux]	2m	🟩 🟩
Repo Tests / Plugin Tests	1m	🟩 🟩
Detect changed files	16s	🟩 🟩
Aggregate Test Results	1s	🟥 🟩

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[TylerJang27]

Good catch! Thanks for the PR!

[NellyWhads]

(rebased but it may have broken something with the CI trigger - apologies)