Use `Vec` for `KeyContext` instead of `SmallVec` (#24916)

In Zed the key context almost always has more than 1 entry, so use of
`SmallVec` is just adding overhead.

In Zed while using the editor this typically has more than 8 entries.
Since `ContextEntry` is 48 bytes, if this were made to be a
`SmallVec<[ContextEntry; 10]>` then it would use 480 bytes on the stack,
which to me seems like a lot to be copying. So, instead opting to just
use `Vec`

Release Notes:

- N/A