Add Svalmel (#2600)

## Description:
This PR adds the Christmas map Svalmel with 5 nations.

Describe the PR.

## 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:

DISCORD_USERNAME
Nikola123

map-generator/main.go

@@ -51,6 +51,7 @@ var maps = []struct {
 	{Name: "pluto"},
 	{Name: "southamerica"},
 	{Name: "straitofgibraltar"},
+	{Name: "svalmel"},
 	{Name: "world"},
 	{Name: "big_plains", IsTest: true},
 	{Name: "half_land_half_ocean", IsTest: true},