shell-integration: respect cursor-style-blink

The `cursor` shell feature always used a blinking bar (beam), often to
the surprise of users who set `cursor-style-blink = false`.

This change extends our GHOSTTY_SHELL_FEATURES format to include either
`cursor:blink` (default) or `cursor:steady` based on cursor-style-blink
when the `cursor` feature is enabled, and all shell integrations have
been updated to use that additional information to choose the DECSCUSR
cursor value (5=blinking bar, 6=steady bar).

I also considered passing a DECSCUSR value in GHOSTTY_SHELL_FEATURES
(e.g. `cursor:5`). This mostly worked well, but zsh also needs the blink
state on its own for its block cursor. We also don't support any other
shell feature cursor configurability (e.g. the shape), so this was an
over generalization.

This does change the behavior for users who like the blinking bar in the
shell but have `cursor-blink-style = false` for other reasons. We could
provide additional `cursor` shell feature configurability, but I think
that's best left to a separate change.

src/termio/shell_integration.zig

@@ -188,11 +188,13 @@ test detectShell {
 pub fn setupFeatures(
     env: *EnvMap,
     features: config.ShellIntegrationFeatures,
+    cursor_blink: bool,
 ) !void {
     const fields = @typeInfo(@TypeOf(features)).@"struct".fields;
     const capacity: usize = capacity: {
         comptime var n: usize = fields.len - 1; // commas
         inline for (fields) |field| n += field.name.len;
+        n += ":steady".len; // cursor value
         break :capacity n;
     };
 
@@ -221,6 +223,10 @@ pub fn setupFeatures(
         if (@field(features, name)) {
             if (writer.end > 0) try writer.writeByte(',');
             try writer.writeAll(name);
+
+            if (std.mem.eql(u8, name, "cursor")) {
+                try writer.writeAll(if (cursor_blink) ":blink" else ":steady");
+            }
         }
     }
 
@@ -241,8 +247,8 @@ test "setup features" {
         var env = EnvMap.init(alloc);
         defer env.deinit();
 
-        try setupFeatures(&env, .{ .cursor = true, .sudo = true, .title = true, .@"ssh-env" = true, .@"ssh-terminfo" = true, .path = true });
-        try testing.expectEqualStrings("cursor,path,ssh-env,ssh-terminfo,sudo,title", env.get("GHOSTTY_SHELL_FEATURES").?);
+        try setupFeatures(&env, .{ .cursor = true, .sudo = true, .title = true, .@"ssh-env" = true, .@"ssh-terminfo" = true, .path = true }, true);
+        try testing.expectEqualStrings("cursor:blink,path,ssh-env,ssh-terminfo,sudo,title", env.get("GHOSTTY_SHELL_FEATURES").?);
     }
 
     // Test: all features disabled
@@ -250,7 +256,7 @@ test "setup features" {
         var env = EnvMap.init(alloc);
         defer env.deinit();
 
-        try setupFeatures(&env, std.mem.zeroes(config.ShellIntegrationFeatures));
+        try setupFeatures(&env, std.mem.zeroes(config.ShellIntegrationFeatures), true);
         try testing.expect(env.get("GHOSTTY_SHELL_FEATURES") == null);
     }
 
@@ -259,9 +265,25 @@ test "setup features" {
         var env = EnvMap.init(alloc);
         defer env.deinit();
 
-        try setupFeatures(&env, .{ .cursor = false, .sudo = true, .title = false, .@"ssh-env" = true, .@"ssh-terminfo" = false, .path = false });
+        try setupFeatures(&env, .{ .cursor = false, .sudo = true, .title = false, .@"ssh-env" = true, .@"ssh-terminfo" = false, .path = false }, true);
         try testing.expectEqualStrings("ssh-env,sudo", env.get("GHOSTTY_SHELL_FEATURES").?);
     }
+
+    // Test: blinking cursor
+    {
+        var env = EnvMap.init(alloc);
+        defer env.deinit();
+        try setupFeatures(&env, .{ .cursor = true, .sudo = false, .title = false, .@"ssh-env" = false, .@"ssh-terminfo" = false, .path = false }, true);
+        try testing.expectEqualStrings("cursor:blink", env.get("GHOSTTY_SHELL_FEATURES").?);
+    }
+
+    // Test: steady cursor
+    {
+        var env = EnvMap.init(alloc);
+        defer env.deinit();
+        try setupFeatures(&env, .{ .cursor = true, .sudo = false, .title = false, .@"ssh-env" = false, .@"ssh-terminfo" = false, .path = false }, false);
+        try testing.expectEqualStrings("cursor:steady", env.get("GHOSTTY_SHELL_FEATURES").?);
+    }
 }
 
 /// Setup the bash automatic shell integration. This works by