Shortcut activator - #78522

This PR introduced a new class, ShortcutActivator, and refactored the the definition and lookup system of shortcuts, in order to solve a few issues of the previous algorithm.

ShortcutActivator is designed to be another base class of LogicalKeySet, and the new standard interface to define a key combination and be mapped to an Intent. LogicalKeySet now implements ShortcutActivator, keeping its previous behavior. However, when defining typical shortcuts such as Delete or Ctrl-C, it's recommended to use the new SingleActivator instead, which behaves more closely to how common OSs do by addressing following issues.

This is part of the general keyboard project as in #44918.

Why

LogicalKeySet as a set of keys does not satisfy the need for properly defining shortcut key combinations. Most typical OSs handle shortcuts by treating modifiers and non-modifiers differently.

• Although modifiers must strictly match the definition (unsided), while non-modifiers allow extra keys. For example, pressing C while holding Ctrl-A also triggers Ctrl-C.
• Also, the last key of the key combination must be the non-modifier key. For example, pressing Ctrl while holding C will not trigger Ctrl-C.

This shows that modifiers and non-modifiers are born unequal, and it's impossible to properly handle them using a set.

Lookup

Some extent of indexing, instead full linear search, is still desired for performance. ShortcutActivator therefore splits requirements into two parts:

• triggers, a set of logical keys, and
• accepts, a predicate function to judge whether the event and keyboard state fully satisty the conditions.

Prompt-intent pairs are indexed by triggers, such as key C as in Ctrl-C, or both A and B as in Alt-A-B. When a trigger key occurs, all activators are tested linearly to find the first matching intent.

Other changes

The algorithm to synchronize modifiers on Web has been change to allow more refined behaviors related to modifiers. More specifically,

• If a modifier key was recorded pressed, but the metaState reports their flag as released, the keys of both sides will be set as released. (Previously: unhandled.)
• If a modifier key was recorded released, but the metaState reports their flag as pressed, the left key will be set as pressed. (Previously: both sides pressed.)
• If a modifier key was recorded pressed, and the metaState reports their flag as pressed too, no synchronization will be added. (Previously: both sides pressed.)

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• All existing and new tests are passing.

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