refactor: use pre-created Set for modifier key lookup in isModifierKey

[TrevorBurnham]

This PR replaces the inline array creation in isModifierKey with a pre-created Set for O(1) lookup, eliminating array allocation on every function call.

Motivation

The isModifierKey function is called on every keydown event via the focus-visible module's handleKeydown handler. The previous implementation created a new array on every call:

return [KeyCode.shift, KeyCode.alt, KeyCode.control, KeyCode.meta].includes(event.keyCode);

This means every keypress triggered:

1. Array allocation for 4 elements
2. Linear search via .includes()

This could contribute to typing feeling sluggish.

Changes

src/internal/keycode.ts: Create the Set once at module load time and use Set.has() for O(1) lookup:

const modifierKeyCodes = new Set([KeyCode.shift, KeyCode.alt, KeyCode.control, KeyCode.meta]);

export function isModifierKey(event: KeyboardEvent) {
  return modifierKeyCodes.has(event.keyCode);
}

Benefits

• No allocation per call (Set created once at module load)
• O(1) lookup instead of O(n) linear search
• Cleaner separation of data and logic

Testing

All 17 focus-visible tests pass, including tests for each modifier key (Shift, Alt, Control, Meta).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (78240a2) to head (92a6979).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #188   +/-   ##
=======================================
  Coverage   99.00%   99.01%           
=======================================
  Files          44       44           
  Lines        1313     1314    +1     
  Branches      363      341   -22     
=======================================
+ Hits         1300     1301    +1     
- Misses         12       13    +1     
+ Partials        1        0    -1     

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