Auto merge of #20327 - kwonoj:fix-ignore, r=paulrouget

fix(keyevent): do not emit default ignorable codepoint

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This PR intends to update `KeyEvent` emit behavior around #18130. Issue https://github.com/servo/servo/issues/17146#issue-233361568 briefly explains what's happening in servo currently - there are KeyboardInput event emitted for separated key (modifier, and `V` in case of paste) and there are also `ReceivedCharacter` event corresponds to `\u{0016}`.

`0x0016` is unicode representation of `Synchronous Idle` (https://en.wikipedia.org/wiki/Synchronous_Idle), belong under category of `Default Ignorable` charater in unicode range doesn't have visual representation (http://www.unicode.org/L2/L2002/02368-default-ignorable.pdf). Currently servo forwards all of emitted event from `winit` including this, eventually leads into double execution of control event.

In this change try to omit default ignorable charater , if given char received is within range of ignorable do not dispatch `KeyEvent`. Once those are omitted, current event handling logic already takes care of key event with correct modifier state so duplicated event handling won't occur.

For implementation perspective, `std::char` in Rust doesn't seem to support `isIdentifierIgnorable` like other platform does (i.e: https://msdn.microsoft.com/en-us/library/aa285330(v=vs.60).aspx / https://docs.oracle.com/javase/7/docs/api/java/lang/Character.html) - so does quick, naïve range comparison check based on unicode range specified in spec, similar to halfbuzz and other does. (https://github.com/harfbuzz/harfbuzz/blob/master/src/hb-unicode-private.hh#L134)

Lastly, this is indeed behavior of `winit` to emit all characters by default. *Why not try to make upstream changes instead?* While I've been reading through issues in `winit`, issue like https://github.com/tomaka/winit/issues/350 trying to emit ignorable character by its intention (delete key `ReceivedCharacter` is also under category of default ignorable) and let each consumer application handles it as needed. I assume it'll cause breaking changes in winit's design if it intends to omit those characters, instead tried to make application-level changes.

Couple of consideration for review

- Is it desired changes to not emit `KeyEvent` for default ignorable chars? Do we rather want mapping / or restoring back to original char as @paulrouget mentioned in https://github.com/servo/servo/issues/17146#issue-233361568?
- Any better, recommended approach to detect unicode char range?
- Maybe try to make upstream changes to `winit` still, like having configuratble way to opt-in(out) those char event?

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #18130 (github issue number if applicable).

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