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

[Eli Zaretskii]

> From: Jimmy Yuen Ho Wong <wyuenho@gmail.com>
> Date: Sun, 04 Jun 2023 00:57:19 +0100
> 
> 
> 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.

Thanks.  CC'ing the relevant developers.