ci: Add shellcheck for scripts (#20631)

Fixes shellcheck errors in script/*
Adds a couple trailing newlines.
Adds `script/shellcheck-scripts` and associated CI machinery.
Current set ultra-conservative, does not output warnings, only errors.

.github/workflows/script_checks.yml

@@ -0,0 +1,21 @@
+name: Script
+
+on:
+  pull_request:
+    paths:
+      - "script/**"
+  push:
+    branches:
+      - main
+
+jobs:
+  shellcheck:
+    name: "ShellCheck Scripts"
+    if: github.repository_owner == 'zed-industries'
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+      - name: Shellcheck ./scripts
+        run: |
+          ./script/shellcheck-scripts error