feat: detect .h files as c filetype (CopilotC-Nvim#1044)

This change adds filetype detection for .h header files to be recognized
as C language files, improving syntax highlighting and language-specific
features for C header files in the CopilotChat plugin.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Closes CopilotC-Nvim#1043

Author: deathbeam

lua/CopilotChat/utils.lua

@@ -339,6 +339,10 @@ function M.filetype(filename)
       return 'sh'
     end
 
+    if vim.endswith(filename, '.h') then
+      return 'c'
+    end
+
     -- weird TypeScript bug for vim.filetype.match
     -- see: https://github.com/neovim/neovim/issues/27265
     if vim.endswith(filename, '.ts') then