Replace var with let

src/display/Display.js

src/display/gutters.js

src/display/highlight_worker.js

src/display/line_numbers.js

src/display/operations.js

src/display/scroll_events.js

src/display/scrollbars.js

src/display/scrolling.js

src/display/selection.js

src/display/update_display.js

src/display/update_lines.js

src/display/view_tracking.js

src/edit/CodeMirror.js

src/edit/commands.js

src/edit/deleteNearSelection.js

src/edit/drop_events.js

src/edit/fromTextArea.js

src/edit/global_events.js

src/edit/key_events.js

src/edit/main.js