TypeScript: support syntax for unknown types

javascript/ql/src/semmle/javascript/TypeScript.qll

@@ -686,6 +686,9 @@ class TypeExpr extends ExprOrType, @typeexpr {
   /** Holds if this is the `object` type. */
   predicate isObjectKeyword() { none() }
 
+  /** Holds if this is the `unknown` type. */
+  predicate isUnknownKeyword() { none() }
+
   /** Gets this type expression, with any surrounding parentheses removed. */
   override TypeExpr stripParens() {
     result = this
@@ -725,6 +728,7 @@ private class KeywordTypeExpr extends @keywordtypeexpr, TypeExpr {
   override predicate isSymbol() { getName() = "symbol" }
   override predicate isUniqueSymbol() { getName() = "unique symbol" }
   override predicate isObjectKeyword() { getName() = "object" }
+  override predicate isUnknownKeyword() { getName() = "unknown" }
 }
 
 /**

javascript/ql/test/library-tests/TypeScript/TypeAnnotations/UnknownTypeExpr.expected

@@ -0,0 +1 @@
+| tst.ts:137:18:137:24 | unknown |

javascript/ql/test/library-tests/TypeScript/TypeAnnotations/UnknownTypeExpr.ql

@@ -0,0 +1,5 @@
+import javascript
+
+from TypeExpr type
+where type.isUnknownKeyword()
+select type

javascript/ql/test/library-tests/TypeScript/TypeAnnotations/tst.ts

@@ -134,3 +134,4 @@ let tupleWithOptionalElement: [number, string, number?];
 let emptyTuple: [];
 let tupleWithRestElement: [number, ...string[]];
 let tupleWithOptionalAndRestElements: [number, string?, ...number[]];
+let unknownType: unknown;