add test that we detect the used type variable in an infer type

javascript/ql/test/query-tests/Declarations/UnusedVariable/UnusedVariable.expected

@@ -5,6 +5,7 @@
 | eval.js:19:9:19:24 | not_used_by_eval | Unused variable not_used_by_eval. |
 | externs.js:6:5:6:13 | iAmUnused | Unused variable iAmUnused. |
 | importWithoutPragma.jsx:1:1:1:27 | import  ... react'; | Unused import h. |
+| interTypes.ts:1:1:1:37 | import  ... where"; | Unused import Bar. |
 | multi-imports.js:1:1:1:29 | import  ... om 'x'; | Unused imports a, b, d. |
 | multi-imports.js:2:1:2:42 | import  ... om 'x'; | Unused imports alphabetically, ordered. |
 | namespaceImportAsType.ts:3:1:3:23 | import  ... om "z"; | Unused import Z. |

javascript/ql/test/query-tests/Declarations/UnusedVariable/interTypes.ts

@@ -0,0 +1,6 @@
+import { Foo, Bar } from "somewhere"; // OK
+
+type FooBar<T> =
+  T extends [infer S extends Foo, ...unknown[]]
+      ? S
+      : never;