bug#63880: 29.0.91; typescript-ts-mode fails to highlight infer keyword

[Jimmy Yuen Ho Wong]

Reproduction:

1. echo 'type Flatten<Type> = Type extends Array<infer Item> ? Item :
Type;' > test.ts
2. C-x C-f test.ts
3. M-x typescript-ts-mode
4. Observe the infer keyword is not highlighted

Expectation:

Typescript has an
[`infer`](https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#inferring-within-conditional-types)
keyword, it should be highlighted.