add a version of `set_file_handler` that accepts a pointer to a `context` parameter

[elder-george]

This will allow to set handlers that do not depend on global data or that construct response dynamically with fewer allocations.

[AlbertShown]

There is already set_context() and get_context() but for e events in binding functions, so users can set a specific context when binding a function.

Check docs for more info: https://webui.me/docs.html#/

Example:

void myFunction(webui_event_t* e) {
    MyContext* foo = (MyContext*) webui_get_context(e);
}

int main() {

    webui_bind(win, "myFunction", myFunction);
    webui_set_context(win, "myFunction", &someContext);
}

If you still need a context for set_file_handler, can you please give us an example of use?