Avoid loading external plugins from the working directory by default

[AzusaSaku]

This changes external plugin discovery to be application-relative by default instead of depending on the process working directory.

Previously, the default Plugins directory was resolved from Environment.CurrentDirectory. This could be surprising when Cpp2IL is launched from a game directory or another untrusted working directory, since external plugin DLL discovery would then depend on that location. Since external plugins are loaded as .NET assemblies, this avoids accidentally treating the working directory as a plugin trust source.

This PR also adds:

• --plugins-dir to load external plugins from an explicit directory
• --no-plugins to skip external plugin loading while still initializing built-in plugins

Absolute plugin paths continue to work, and relative plugin paths are resolved from the Cpp2IL application directory.