🐛 Bug: isPropertyReadonlyInType crashes on a mapped type parameter extending any

[JoshuaKGoldberg]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Expected

isPropertyReadonlyInType shouldn't crash if provided a type that doesn't have a modifiersType property. Per ajafff/tsutils#136 (comment) that's actually possible.

type MyType<T> = {
  [K in keyof T]: 'cat' | 'dog' | T[K];
};

function method<A extends any[] = string[]>(value: MyType<A>) {
  return value;
}

Actual

Per typescript-eslint/typescript-eslint#3405:

TypeError: Cannot read properties of undefined (reading 'flags')
 ❯ Module.isUnionType src/types/typeGuards.ts:39:15
     37| 
     38| export function isUnionType(type: ts.Type): type is ts.UnionType {
     39|  return (type.flags & ts.TypeFlags.Union) !== 0;
       |               ^
     40| }
     41| 
 ❯ unionTypeParts src/types/utilities.ts:243:9
 ❯ isPropertyReadonlyInType src/types/utilities.ts:132:24
 ❯ isReadonlyPropertyFromMappedType src/types/utilities.ts:98:32
 ❯ src/types/utilities.ts:61:12
 ❯ someTypePart src/types/utilities.ts:218:55
 ❯ isReadonlyPropertyIntersection src/types/utilities.ts:52:9
 ❯ Module.isPropertyReadonlyInType src/types/utilities.ts:146:4

Additional Info

Reproduction steps:

1. Clone https://github.com/typescript-eslint/typescript-eslint locally
2. Switch to its v6 branch and run yarn
3. Add that test case to prefer-readonly-parameter-types.test.ts
4. Run yarn jest require-await in packages/eslint-plugin