callback: handle callback parameter on constructor

[krichprollsch]

Adding a Callback parameter on a constructor failed with:

$ zig build                                                                                                                                  
install                                                                                                                                      
└─ install browsercore                                                                                                                       
   └─ zig build-exe browsercore Debug native 1 errors                                                                                        
vendor/zig-js-runtime/src/engines/v8/generate.zig:980:73: error: overflow of integer type 'usize' with value '-1'                            
        const js_func_index = func.callback_index.? - func.index_offset - 1;                                                                 
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~                                                                                                                                                                                                               
referenced by:                                                                                                                               
    constructor: vendor/zig-js-runtime/src/engines/v8/generate.zig:999:21                                                                    
    generateConstructor__anon_19702: vendor/zig-js-runtime/src/engines/v8/generate.zig:1007:6                                                
    remaining reference traces hidden; use '-freference-trace' to see all reference traces                                                   

This PR:

• adds a kind to Func struct
• uses kind to fix callback offset creation