Add shellcheck

This GH action checks (in every commit to main) the shell scripts. Will only flag `severity: error` level problems as failures. Feel free to add `warnings` as error too if you need so.

.github/workflows/pre-main.yaml

@@ -0,0 +1,22 @@
+name: Test Incoming Changes
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+  workflow_dispatch:
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@master
+        env:
+          SHELLCHECK_OPTS: -e SC2068
+        with:
+          ignore_names: bashrc
+          severity: error