Adds onion map (#4057)

If this PR fixes an issue, link it below. If not, delete these two
lines.
Resolves #4055 

## Description:

Adds a 512*512 onion map with 3 nations (Leafer Confederation, Outer
Enclave and Inner Tribe)

<img width="128" height="128" alt="thumbnail"
src="https://github.com/user-attachments/assets/8d97d8dc-6286-4e79-a459-767c936d49ec"
/>


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

tktk1234567

map-generator/assets/maps/onion/info.json

@@ -0,0 +1,20 @@
+{
+  "name": "onion",
+  "nations": [
+    {
+      "coordinates": [51, 188],
+      "name": "Leafer Confederation",
+      "flag": ""
+    },
+    {
+      "coordinates": [252, 199],
+      "name": "Outer Enclave",
+      "flag": ""
+    },
+    {
+      "coordinates": [252, 233],
+      "name": "Inner Tribe",
+      "flag": ""
+    }
+  ]
+}

map-generator/main.go

@@ -83,6 +83,7 @@ var maps = []struct {
 	{Name: "northamerica"},
 	{Name: "northwestpassage"},
 	{Name: "oceania"},
+	{Name: "onion"},
 	{Name: "pangaea"},
 	{Name: "passage"},
 	{Name: "pluto"},