fix: correctly detect shift key only for uppercase letters

[benny123tw]

Description

When I press the return key (\r) on macOS, the key object incorrectly indicates that the shift key is also pressed:

{
  input: '\r',
  key: {
    upArrow: false,
    downArrow: false,
    leftArrow: false,
    rightArrow: false,
    pageDown: false,
    pageUp: false,
    return: true,
    escape: false,
    ctrl: false,
    shift: true,
    tab: false,
    backspace: false,
    delete: false,
    meta: false
  }
}

Root Cause

The previous logic inferred the shift key was pressed for any single character where input[0].toUpperCase() === input[0]. This included not only uppercase letters, but also control characters (like \r) and symbols, leading to incorrect detection of the shift key.

Solution

This PR updates the shift key detection logic to use a regular expression (/^[A-Z]$/) that matches only uppercase ASCII letters. Now, key.shift will only be set to true when the input is an uppercase letter, and not for control characters or symbols.

Testing

• Verified that pressing the return key no longer sets key.shift to true.
• Confirmed that uppercase letter input (e.g., Q) still sets key.shift to true.