Ignore Shift when comparing character keys

[ulyssa]

Someone reported issues with pressing : on Windows over at ulyssa/iamb#109. The issue there is that modifiers contains KeyModifiers::SHIFT on Windows, but not on other platforms, because of differences in how crossterm constructs the value. Since SHIFT isn't important for comparing KeyCode::Char keypresseses, I should be able to just remove it when constructing TerminalKey.