improvement(vscode): stop C/C++ changing settings

By default, VS Code's C/C++ extension updates .vscode/settings.json to associate
.h files with C or C++ based on which type of file they're included by. Since
ZMK doesn't use C++, disable this behavior and always treat .h files as C
headers.

.vscode/settings.json

@@ -1,7 +1,9 @@
 {
+  "C_Cpp.autoAddFileAssociations": false,
   "files.associations": {
     "*.overlay": "dts",
-    "*.keymap": "dts"
+    "*.keymap": "dts",
+    "*.h": "c"
   },
   "[c]": {
     "editor.formatOnSave": true