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.

script/get-crate-version

@@ -2,7 +2,7 @@
 
 set -eu
 
-if [[ $# < 1 ]]; then
+if [[ $# -ne 1 ]]; then
   echo "Usage: $0 <crate_name>" >&2
   exit 1
 fi
@@ -14,4 +14,4 @@ cargo metadata \
     --format-version=1 \
     | jq \
         --raw-output \
-        ".packages[] | select(.name == \"${CRATE_NAME}\") | .version"
+        ".packages[] | select(.name == \"${CRATE_NAME}\") | .version"