core: add Surface.stopProcess

This keeps the IO backend interaction inside of the owning Surface code.

src/Surface.zig

@@ -801,6 +801,14 @@ pub fn close(self: *Surface) void {
     self.rt_surface.close(self.needsConfirmQuit());
 }
 
+/// Stop the child process by sending it the kill command (SIGHUP).
+/// child_exited will be set once the child process has stopped.
+pub fn stopProcess(self: *Surface) void {
+    switch (self.io.backend) {
+        .exec => |*exec| exec.subprocess.stop(),
+    }
+}
+
 /// Returns a mailbox that can be used to send messages to this surface.
 inline fn surfaceMailbox(self: *Surface) Mailbox {
     return .{

src/apprt/embedded.zig

@@ -1544,9 +1544,7 @@ pub const CAPI = struct {
 
     /// Stops the child process by sending it the kill command (SIGHUP).
     export fn ghostty_surface_process_stop(surface: *Surface) void {
-        switch (surface.core_surface.io.backend) {
-            .exec => |*exec| exec.subprocess.stop(),
-        }
+        surface.core_surface.stopProcess();
     }
 
     /// Returns true if the surface has a selection.