Add Slang

[davidar]

Description

This adds support for the Slang shading language.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.slang+shader
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/shader-slang/slang/tree/master/examples/autodiff-texture
    • Sample license(s): Apache 2.0 https://github.com/shader-slang/slang/blob/master/LICENSE
  • I have included a syntax highlighting grammar: https://github.com/shader-slang/slang-vscode-extension
    • I've opened a PR for the issues reported by the grammar compiler: Fix validation errors in syntax definition shader-slang/slang-vscode-extension#18
  • I have added a color
    • Hex value: #1fbec9
    • Rationale: I selected one of the two main colours from the Slang logo.
  • I have updated the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Important

The changes in this PR will not appear on GitHub until the next release has been made and deployed. See here for more details.