Support SpringBoot nested jar file

When we use pkl config in Java SprintBoot application
We will get error
```
Caused by: org.pkl.core.PklException: –– Pkl Error ––
Refusing to load module `jar:nested:/app.jar/!BOOT-INF/classes/!/config/dev.pkl` because it does not match any entry in the module allowlist (`--allowed-modules`).

```

it's because jar:nested isn't include in allow-modules.

we could fix it by add it into allowModules and change defaultTrustLevel, but how about add jar:nested into java default configuration?

Start by reproducing the Spring Boot nested-jar error and reviewing how allowModules and defaultTrustLevel handle the jar:nested URL scheme. Done means deciding whether the Java default configuration should support this scheme and covering that decision with appropriate tests.

Written by the indexing model from the issue text.