fix(terminal): keep macOS dead keys out of the PTY until composed

[ErikZigo]

• Fixes typing accented letters that need a dead key on macOS — Czech ď, ť, ň and U.S. é, ô now come out as one character instead of a stray accent followed by the letter (ˇď)
• Shows the pending accent underlined at the cursor while composing, the way Terminal.app and iTerm2 do
• Nothing reaches the shell until a character is actually committed, so an abandoned composition leaves the terminal clean

Impact

• Affects everyone typing dead-key accents on macOS — Czech, Slovak, German, French, Spanish and Portuguese layouts in particular
• Correct text input for non-English layouts, which previously produced garbage on every dead key
• Only the path that receives composed text changes; plain keystrokes, shortcuts and paste are untouched