feat(60475): Add getUnknownType to checker api (#60502)

Author: a-tarasyuk

src/compiler/checker.ts

@@ -1824,6 +1824,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         getNumberLiteralType,
         getBigIntType: () => bigintType,
         getBigIntLiteralType,
+        getUnknownType: () => unknownType,
         createPromiseType,
         createArrayType,
         getElementTypeOfArrayType,

src/compiler/types.ts

@@ -5263,6 +5263,7 @@ export interface TypeChecker {
     getBigIntType(): Type;
     getBigIntLiteralType(value: PseudoBigInt): BigIntLiteralType;
     getBooleanType(): Type;
+    getUnknownType(): Type;
     /* eslint-disable @typescript-eslint/unified-signatures */
     /** @internal */
     getFalseType(fresh?: boolean): Type;

tests/baselines/reference/api/typescript.d.ts

@@ -6288,6 +6288,7 @@ declare namespace ts {
         getBigIntType(): Type;
         getBigIntLiteralType(value: PseudoBigInt): BigIntLiteralType;
         getBooleanType(): Type;
+        getUnknownType(): Type;
         getFalseType(): Type;
         getTrueType(): Type;
         getVoidType(): Type;