[skrifa] Feature Request: Add `font.is_variable()`

Something like:

```rs
fn is_variable(&self) -> bool {
self.fvar().is_ok()
}
```

or

```rs
fn is_variable(&self) -> bool {
self.axes().len() > 0
}
```

Start by locating the skrifa Font API and its existing fvar() and axes() entry points. Determine which behavior should define is_variable(), then add the method and verify it against variable and non-variable fonts using the repository's relevant tests, if available.

Written by the indexing model from the issue text.