Handle IME-transformed printable input

[yasuhito]

Fixes #5348

This changes printable key handling to give beforeinput/input a chance to provide the authoritative text before xterm.js sends the raw keydown character.

Some IMEs such as SKK commit certain characters without a compositionstart/compositionend sequence. In Chromium-based runtimes, a key like a can arrive as:

• keydown: raw key a
• beforeinput/input: committed IME text あ

Before this change, xterm.js sent the raw keydown text immediately, so terminal apps received a instead of あ. Multi-key SKK input such as ka can still work because the first key enters IME state and the committed text arrives later.

The fix:

• defers unmodified, single-character printable keydown data for one tick
• handles beforeinput first, preventing the DOM mutation when possible
• falls back to input if beforeinput is unavailable
• flushes the original keydown text if no text input event arrives
• leaves Kitty keyboard protocol, Win32 input mode, modified keys, control sequences, and screen reader mode on the existing synchronous path

Added unit coverage for:

• IME-transformed beforeinput data replacing raw printable keydown data (a -> あ)
• regular printable text still being sent (a -> a)
• fallback when no beforeinput/input arrives
• special keyboard modes not being deferred

Validation run locally:

• npm run build
• npm run esbuild
• node ./bin/test_unit.js out-esbuild/browser/Terminal.test.js
• npm run test-unit
• npm run lint-changes