Add Language: USD

## Language name
[USD](https://www.pixar.com/usd), developed by Pixar Animation Studios

## URL of example repository
https://github.com/PixarAnimationStudios/USD - [Example](https://github.com/PixarAnimationStudios/USD/blob/3b097e3ba8fabf1777a1256e241ea15df83f3065/pxr/usd/usdMtlx/testenv/testUsdMtlxFileFormat.testenv/baseline/NodeGraphs.usda)

https://github.com/ColinKennedy/USD-Cookbook - See any `**/usda` folder for examples. [Here's an example](https://github.com/ColinKennedy/USD-Cookbook/blob/master/features/asset_info/usda/asset_info.usda)

## URL of syntax highlighting grammar
### Tree-sitter
https://github.com/ColinKennedy/tree-sitter-usd

### VSCode
https://github.com/AnimalLogic/AL_usd_vscode_extension

### Pygments
[General Syntax](https://github.com/pygments/pygments/blob/a1361035ec5f599290371f7df9f21e3f1f1420a6/pygments/lexers/usd.py)
[List Of Keywords](https://github.com/pygments/pygments/blob/a1361035ec5f599290371f7df9f21e3f1f1420a6/pygments/lexers/_usd_builtins.py)
Full disclosure, I wrote this lexer and it isn't perfect (for example, not catching `.connect` as part of an attribute name). [Screenshot of what the looks like, here](https://github.com/github/linguist/assets/10103049/736b2584-7f1e-45c9-9e08-bc6e00da2094)

### Text Editors
[A simpler, regex-based example](https://github.com/superfunc/usda-syntax)
Vim's example some small issues in my experience but it works pretty well overall

## Most popular extensions
- [.usd](https://github.com/search?q=extension%3Ausd) (7K+)
- [.usda](https://github.com/search?q=extension%3Ausda) (13K+)
- [.usdc](https://github.com/search?q=extension%3Ausdc) (3.9K+)

## Grammar Features
[This gist](https://gist.github.com/ColinKennedy/1997d1f0850031d22245f529cd92872f) shows a fairly complex USD file and some of its syntax features.

## Detected language
None

Start by reviewing existing language-addition entry points and tests in Linguist, then compare the USD and USDA examples with the linked Tree-sitter, VSCode, and Pygments grammars. Done means USD-related files are detected and highlighted consistently for the listed extensions, with coverage for representative syntax.

Written by the indexing model from the issue text.