Add .clangd as a filename for YAML

[tahmid-khan]

This PR classifies .clangd as a name for files that use the YAML language.

Description

clangd uses YAML-formatted files with the name .clangd for its project-level configuration.

Reference: clangd’s documentation for configuration files

Also potentially of interest: JSON Schema specification for .clangd files

Checklist:

• I am adding a new extension to a language.
  • The new extension 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.clangd+CompileFlags
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s): I wrote the sample file based on some options I commonly use.
    • Sample license(s): MIT License
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.