Auto-detect file extensions instead of requiring manual config

[mircealungu]

Problem

Users must manually specify `fileExtensions` in the config (e.g. `[".cs"]`, `[".java"]`). If omitted, the tool defaults to Python parsing. This is unintuitive — the tool should be able to scan the source directory and detect the language automatically.

Suggestion

When `fileExtensions` is not specified, scan `rootFolder` for source files and auto-detect:

• `.cs` files → C#
• `.java` files → Java
• `.go` files → Go
• `.kt` files → Kotlin
• `.py` files → Python

This would make `archlens init && archlens render` work without manual config editing.