proposal: make goplus/lib version configurable instead of hardcoded

Requested by @luoliwoshang in #612

Current Situation

The goplus/lib version is currently hardcoded in cl/internal/convert/convert.go:46:

loadDeps := []string{"github.com/goplus/lib@v0.3.1"}

Problem

Having the version hardcoded makes it difficult to:

1. Update the version without modifying source code
2. Allow users to specify their preferred version
3. Test with different versions of goplus/lib
4. Maintain backwards compatibility when needed

Proposed Solution

Consider making the goplus/lib version configurable through:

• Configuration file option in llcppg.cfg
• Command-line flag (e.g., --goplus-lib-version)
• Environment variable
• Default to latest stable version if not specified

Benefits

• More flexible for users with different requirements
• Easier to update without code changes
• Better support for testing and development
• Cleaner separation of configuration from code

Related

• PR #612 updated the hardcoded version from v0.2.0 to v0.3.1
• Issue #611 requested the version update

This issue is for discussing and implementing a more flexible approach to version management.

Start at cl/internal/convert/convert.go:46 and inspect how llcppg.cfg and command-line options are currently handled. Decide which configuration path this proposal should use and define how the default and user-selected goplus/lib versions will be tested; done means the hardcoded version is no longer the only option.