fix(modifier key): reset modifier keys when browser tab loses focus/is hidden

[jbocce]

Context

Fixes #733.

Changes & Results

In packages\tools\src\eventListeners\keyboard\keyDownListener.ts, reset the modifier key whenever the browser tab is 'hidden' (i.e. loses focus). Prior to this PR, the modifier key for changing tab (e.g. Ctrl) or app (e.g. Alt) focus would remain in the modifier key state and was never cleared until a subsequent key-up event.

Testing

First and foremost, ensure issue #733 is resolved.

Test A:

1. Run the doubleClickWithStackAnnotationTools example.
2. Add various annotations.
3. Either Ctrl-tab or Alt-tab to change the tab/app focus.
4. In the destination app/window/tab perform some mouse clicks/wheels and/or key strokes.
5. Return to the tab running the example.
6. One should still be able to add and edit annotations.

Test B:

1. Run the modifierkeys example.
2. Exercise the various aspects of the example.
3. Either Ctrl-tab or Alt-tab to change the tab/app focus.
4. In the destination window/tab perform some mouse clicks/wheels and/or key strokes.
5. Return to the tab running the example.
6. The example should continue to behave as before.

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

• I have run the yarn build:update-api to update the API documentation, and have
  committed the changes to this PR. (Read more here https://www.cornerstonejs.org/docs/contribute/update-api)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• "OS: Windows 11
• "Node version: 16.14.0
• "Browser: Chrome 116.0.5845.111

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