Improvements to collection keyboard navigation

[wolfgang42]

Change

Fix collisions between browser shortcuts and comic next/previous navigation. See commit messages for full details.

Bug Fix – Fixes an issue with existing functionality

Context

Previously, keyboard modifiers (Ctrl/Alt/Meta) were ignored when looking up keyboard shortcuts. This made it impossible to use the browser's native ArrowLeftAlt/ArrowRightAlt shortcuts for history navigation, since the plugin would also perform navigation of its own on the same shortcuts.

Verification

• [x] I have read the code of conduct and contributing guidelines
• [x] I have followed the project's coding standards
• [x] ~~I have added tests to cover all changes~~
• [x] ~~I have verified that all new and existing tests pass~~ (N/A, there don't seem to be tests for JS)
• [x] I have tested these changes on WordPress 5.7 (as Vagrant provisioning seems to be broken, tested by replacing wp-content/plugins/webcomic/srv/collection/common.js with an unminified, modified version)
• [x] I have tested these changes on PHP 7.4.16
• [x] I have tested these changes in: Firefox 86, Chromium 89

[wolfgang42]

@mgsisk bump - LMK if there's anything you need from me on this