feat: support keyboard system modifiers

[iamdhrv]

Change

• add .ctrl, .alt, .shift, .meta, and .cmd system-key modifiers
• require every requested system modifier before dispatching the handler
• cover positive and negative cases for each modifier
• cover a chained @keydown.shift.enter handler

Why

EventBinder already filters named keys such as .enter, but it had no way to
express common keyboard shortcuts. Mapping modifiers to the standard event
flags keeps this logic local to the existing modifier pipeline and allows
system and named-key filters to compose.

Checks

• node test/unit/eventModifiers.test.js
• npm run lint -- --quiet
• npm test: 69/71 test files passed; the two unrelated failures require
  writing user-level files outside the repository sandbox

Fixes #637

[nathanschmid08]

Hi @iamdhrv , Thank you so much for the good work. I really appreciate it. Everything looks good, great job! I will mark the draft as ready for review and then merge it.
Thanks.