Add shellcheck lint

Run shellcheck [1] on the bash files.

[1]: https://www.shellcheck.net/

Signed-off-by: Tim Crawford <tcrawford@system76.com>

scripts/lint/03-shellcheck.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-3.0-only
+
+# Check if any shell scripts have issues.
+
+LINT_DIR=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &> /dev/null && pwd)
+. "$LINT_DIR/util.sh"
+
+echo -n "Checking shell scripts..."
+
+if ! command -v shellcheck > /dev/null; then
+    skipped "shellcheck not found"
+    exit 0
+fi
+
+readarray -t FILES < <(git ls-files '*.sh')
+needs_formatting=()
+
+for file in "${FILES[@]}"; do
+    # SC1091: Ignore external scripts
+    if ! shellcheck -f quiet --exclude=SC1091 "$file"; then
+        needs_formatting+=("$file")
+    fi
+done
+
+if [[ "${#needs_formatting[@]}" != "0" ]]; then
+    failed
+
+    for file in "${needs_formatting[@]}"; do
+        echo "- $file"
+    done
+
+    exit 1
+fi
+
+passed