Spring.GetFactoryCounts returns weird format

Here's an example of the output:

```lua
{
[2]={
[260]=20,
},
[3]={
[300]=5,
},
n=3,
}
```

This is very unergonomic and unexpected to work with. Maybe add an argument to not return n for retrocompat concerns, if there are any.

#### Workaround

```lua
-- NOTE: Engine for some reason returns this weird format:
-- ```
-- {
-- [2]={
-- [260]=20,
-- },
-- [3]={
-- [300]=5,
-- },
-- n=3,
-- }
-- ```
-- See: https://github.com/beyond-all-reason/RecoilEngine/issues/2571
local function getNonBrokenEngineReturn(brokenReturn)
if not brokenReturn then
return brokenValue
end

local value = {}
for brokenKey, brokenVal in pairs(brokenReturn) do
if brokenKey ~= "n" then
local actualKey, actualVal = next(brokenVal)

if actualKey then
value[actualKey] = actualVal
end
end
end

return value
end

local notUsableFactoryCounts = Spring.GetFactoryCounts(unitID)
local usableFactoryCounts = getNonBrokenEngineReturn(notUsableFactoryCounts)
```

Start at the Spring.GetFactoryCounts API entry point and compare its returned table with the example and the getNonBrokenEngineReturn workaround in this issue. Resolve whether the existing n field must remain for compatibility or an opt-out is needed; done when the returned factory counts are ergonomic without breaking the required compatibility behavior.

Written by the indexing model from the issue text.