Move map generator into main repo (#2006)

## Description:

Move the MapGenerator repo into OpenFrontIO so we don't need to copy
generated map files over.

## 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:

evan

map-generator/generated/test_maps/big_plains/manifest.json

@@ -0,0 +1,13 @@
+{
+  "map": {
+    "height": 200,
+    "num_land_tiles": 40000,
+    "width": 200
+  },
+  "mini_map": {
+    "height": 100,
+    "num_land_tiles": 10000,
+    "width": 100
+  },
+  "name": "Big Plains"
+}