Add ICU syntax highlighting with XML

### Describe the issue

In [`icu-message-format` docs](https://docs.weblate.org/en/weblate-4.13.1/user/checks.html#icu-messageformat), there are useful `icu-flags` mentioned, but there is no explanation on how to use it.

I've tried `icu-message-format,icu-flags:xml`


but that didn't seem to work (icu syntax got highlighted, but XML tags didn't):


### I already tried

- [X] I've read and searched [the documentation](https://docs.weblate.org/).
- [X] I've searched for similar issues in this repository.

### Steps to reproduce the behavior

–

### Expected behavior

_No response_

### Screenshots

_No response_

### Exception traceback

_No response_

### How do you run Weblate?

Docker container

### Weblate versions

4.13.1

### Weblate deploy checks

_No response_

### Additional context

_No response_