add: .line core event for line contents in non *raw mode* instead of .key core events

The end of the `.line` event received contents is highlighted by a trailing newline
(which cannot occur before, as that triggers the line event itself).

Add signal handler for SIGCONT which forces a `.resize` event that should re-draw the
contents after continuing a suspended application (i.e. ctrl+z followed by `fg`).

src/event.zig

@@ -23,6 +23,8 @@ pub const SystemEvent = union(enum) {
         /// associated error message
         msg: []const u8,
     },
+    /// Input line event received in non *raw mode* (instead of individual `key` events)
+    line: []const u8,
     /// Input key event received from the user
     key: Key,
     /// Mouse input event