Fix env for lua5.1/luajit

[espkk]

Basically adds setfenv/getfenv support for lua5.1/luajit

[gemini-code-assist]

Code Review

This pull request introduces support for Lua 5.1 function environments (fenv) by updating the evaluation scripts, variable handling, and the C++ backend's reference value system. Feedback suggests refactoring the environment detection logic in variables.lua to eliminate redundant assignments for the default global environment.

[gemini-code-assist]

The logic for setting the default global environment (_G) is duplicated in both the else branch of the Lua 5.1 check and the else branch of the custom environment check. This can be simplified by setting the defaults first and then overriding them only if the specific Lua 5.1 condition is met, which improves maintainability.

        eval = "_G"
        value = rdebug._G
        if LUAVERSION == 51 then
            local fenv = rdebug.getfenv(info.func)
            if fenv and not rdebug.equal(fenv, rdebug._G) then
                eval = nil
                value = fenv
            end
        end