kong.JSON doesn't load camelCase values in embedded structs

Hi,

I haven't dug into this much, but I was seeing some strange behavior when trying to load values from a json file when using embedded structs.

for some reason, camelCase keys in the json file do not get picked up when they are used in embedded structs.

The following testcase shows the problem:
```

func TestConfigReallyLoadsValues(t *testing.T) {
type Level struct {
Flag string `json:"flag" required:""`
WithSnake string `json:"with_snake,omitempty" required:""`
WithCamel string `json:"withCamel,omitempty" required:""`
}
var cli struct {
TopSnake string `json:"top_snake" required:""`
TopCamel string `json:"topCamel" required:""`
Level `json:"level" prefix:"level." embed:""`
}

cli.TopCamel = "filled"
cli.TopSnake = "filled"
cli.Level.WithCamel = "filled"
cli.Level.WithSnake = "filled"
cli.Level.Flag = "filled"
conf, cleanConf := makeConfig(t, &cli)
defer cleanConf()

b, err := os.ReadFile(conf)
assert.NoError(t, err)
log.Printf("data: %s", b)

p := mustNew(t, &cli, kong.Configuration(kong.JSON, conf))
_, err = p.Parse(nil)
assert.NoError(t, err)
}
```

```
=== RUN TestConfigFlag
--- PASS: TestConfigFlag (0.00s)
=== RUN TestConfigValidation
--- PASS: TestConfigValidation (0.00s)
=== RUN TestConfigReallyLoadsValues
2025/01/21 22:51:11 data: {"top_snake":"filled","topCamel":"filled","level":{"flag":"filled","with_snake":"filled","withCamel":"filled"}}
config_test.go:72: Did not expect an error but got:
missing flags: --level.with-camel=STRING
--- FAIL: TestConfigReallyLoadsValues (0.00s)
FAIL
FAIL github.com/alecthomas/kong 0.003s
FAIL
```

Top level keys expressed as snake or camel are properly parsed, but for the embedded struct, only the snake case version is picked up.