fix(tauri): don't remove once listener inside async task

[chippers]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Spawning an async task to remove the once listener caused it to be able to be called multiple times before being removed. This design choice was previously made due to deadlock happening when removing the event from inside event::Listeners::once(/*...*/). That was because the listeners were already locked inside trigger when asked to be removed. trigger now handles removing once handlers, preventing a second lock attempt.

• Window::trigger is now pub not pub(crate).
• Adding EventHandler return to once and once_global could be considered breaking if the result of the expression (previously ()) was used directly instead of adding a semicolon. The fix in downstream code affected by it is adding a semicolon.

IIRC, the fn once deadlock fix was happening when calling trigger. If there are more cases in which it was happening that this doesn't fix, let me know.