Add modal input state and rails

src/main.zig

@@ -491,6 +491,7 @@ test "regular: local editor client exposes save and dirty quit guards" {
     defer std.testing.allocator.free(initial);
     try std.testing.expectEqualStrings("", initial);
 
+    try client.handleInput("i");
     try client.handleInput("h");
     try client.handleInput("e");
     try client.handleInput("l");
@@ -500,12 +501,19 @@ test "regular: local editor client exposes save and dirty quit guards" {
     defer std.testing.allocator.free(edited);
     try std.testing.expectEqualStrings("hello", edited);
 
+    try client.handleInput(" ");
+    try std.testing.expectEqualStrings("insert_space", client.pendingRailName());
+    try client.handleInput("n");
+    try std.testing.expectEqualStrings("normal", client.modeName());
     try client.handleInput(" ");
     try client.handleInput("s");
     try std.testing.expectEqualStrings("hello", try client.saved());
 
+    try client.handleInput("i");
     try client.handleInput("!");
     try client.handleInput(" ");
+    try client.handleInput("n");
+    try client.handleInput(" ");
     try client.handleInput("q");
     try std.testing.expect(client.requestedQuit());
     client.clearQuit();