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 held key will prevent the new key from repeating. More details in #4061.

Approach

IMO, the key up event shouldn't clear ime_last_event nor should it flip the ime_state (since the event is generally associated with key down events). Hence I'm adding a simple check in key_common to prevent the event from being set 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.

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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.