Meta: Add gn linter

This just calls ``gn format`` on any .gn or .gni files, but it matches
the pattern of the other linters.

Meta/lint-gn.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+
+set -e
+
+script_path=$(cd -P -- "$(dirname -- "$0")" && pwd -P)
+cd "${script_path}/.." || exit 1
+
+if [ "$#" -eq "0" ]; then
+    mapfile -t files < <(
+        git ls-files '*.gn' '*.gni'
+    )
+else
+    files=()
+    for file in "$@"; do
+        if [[ "${file}" == *".gn" ]] || [[ "${file}" == *".gni" ]]; then
+            files+=("${file}")
+        fi
+    done
+fi
+
+if (( ${#files[@]} )); then
+    if ! command -v gn >/dev/null 2>&1 ; then
+        echo "gn is not available, but gn files need linting! Either skip this script, or install gn."
+        exit 1
+    fi
+    gn format "${files[@]}"
+else
+    echo "No .gn or .gni files to check."
+fi