Add map-generator --maps flag to process a subset of maps (#2595)

Resolves #2594

## Description:

- Added an optional `--maps` flag to the map-generator that allows
passing a subset of maps to process.
- Updated `README.md` w/ documentation for using the new flag

Used the [go/flag](https://pkg.go.dev/flag) package.

Running without the flag, or with no arguments matches existing behavior
of processing all the maps defined in `main.go`

`go run .`
`go run . --maps=`

New behavior is triggered to process a subset of maps by passing the
`--maps` flag. Multiple maps are defined in a comma-separated list

`go run . --maps=world,africa`

Or a single map (useful for map development)

`go run . --maps=fourislands`

If an invalid map is passed, the process aborts and renders the error

```bash
go run . --maps=invalidmapname
Error generating terrain maps: map "invalidmapname" is not defined
exit status 1
```

By using the default flag package, we get `--help` for free:

```bash
go run . --help
Usage of <path-to-exe>/exe/map-generator:
  -maps string
    	optional comma-separated list of maps to process. ex: --maps=world,eastasia,big_plains
```

I do not write GO often.  Gemini contributed to this syntax.

## 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:

tidwell

map-generator/README.md

@@ -17,6 +17,16 @@ This is a tool to generate map files for OpenFront.
 5. Run the generator: `go run .`
 6. Find the output folder at `../resources/maps/<map_name>`
 
+By default, this will process all defined maps.
+
+Use `--maps` to process a single map:
+
+`go run . --maps=fourislands`
+
+To process a subset of maps, pass a comma-separated list:
+
+`go run . --maps=northamerica,world`
+
 ## Create image.png
 
 1. [Download world map (warning very large file)](https://drive.google.com/file/d/1W2oMPj1L5zWRyPhh8LfmnY3_kve-FBR2/view?usp=sharing)