fix: don't clear ime_last_event when a key is UP

[psyclaudeZ]

Issue

When certain keys [1] are pressed and held before a previously held key is released, the key up event from the previously held key will prevent the new key from repeating. More details in #4061.

Approach

The immediate reason of this is that the key up event clears ime_last_event which subsequently clears ime_state hence breaks the logic guarded by this comment:

// ... if the current event
// is a repeat, and the IME previously generated
// `Acted`, we will assume that we're safe to replay
// that last action.

Since, IMO, IME events are generally associated with key being pressed down, I'm adding a simple check in key_common to prevent the event from being cleared by a key up event.

Technically there're other places that clear ime_last_event, but I'm limiting my change to the immediate blast radius of the issue for simplicity.

Test Plan

1. cargo test --all
2. Verified the said issue wouldn't happen anymore by holding k -> holding j -> releasing k.

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[1] - b, f, j, m, p, q, v, x to be exact. The accent menu won't be loaded for these characters on macOS anyway.