TS: Support readonly type expressions

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

@@ -970,6 +970,14 @@ class RestTypeExpr extends @resttypeexpr, TypeExpr {
   TypeExpr getElementType() { result = getArrayType().(ArrayTypeExpr).getElementType() }
 }
 
+/**
+ * A type of form `readonly T`, such as `readonly number[]`.
+ */
+class ReadonlyTypeExpr extends @readonlytypeexpr, TypeExpr {
+  /** Gets the type `T` in `readonly T`. */
+  TypeExpr getElementType() { result = getChildTypeExpr(0) }
+}
+
 /**
  * A possibly qualified name that refers to a variable from inside a type.
  *

javascript/ql/src/semmlecode.javascript.dbscheme

@@ -573,6 +573,7 @@ case @typeexpr.kind of
 | 33 = @optionaltypeexpr
 | 34 = @resttypeexpr
 | 35 = @bigintliteraltypeexpr
+| 36 = @readonlytypeexpr
 ;
 
 @typeref = @typeaccess | @typedecl;