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Description

This PR fixes an inconsistency in useThrottleFn's behavior when leading: false, making it behave more like lodash.throttle.

Previously, when leading was set to false, the function could still execute on the second call if the first call was suppressed. This was because lastExec was not updated when the call was skipped, resulting in incorrect elapsed calculation.

This PR updates the logic to ensure that the throttle interval is respected properly, even when leading: false. This change aligns the behavior with developers’ expectations and lodash's semantics.

Fixes #4920.

Additional context

• The issue arises because VueUse treats the “last execution time” as the moment when the function actually runs, rather than when it is called.
• In contrast, Lodash updates the timestamp immediately when the throttled function is invoked (even if suppressed).
• This led to confusion when leading: false, as the second call would unexpectedly pass through the throttle.
• Test cases have been adjusted to validate the new behavior.

Original issue discussion: [#4920](#4920)