Add test or example or docs for sending a hash map from Lua to Bevy

[mgi388]

Given a Lua function like this:

-- Global state
local my_state = {
    vars = {},        
    a_string = nil, 
}

function get_state()
    local type = world.get_type_by_name("MyResource")
    local my_resource = world.get_resource(type)
    my_resource.a_string = my_state.a_string
    -- Show how to send a table to Bevy.
    -- my_resource.vars = my_state.vars
    -- my_resource.vars = { foo = "bar" }
end

And the resource in Bevy:

#[derive(Debug, Default, Reflect, Resource)]
#[reflect(Debug, Default, Resource)]
pub struct MyResource {
    a_string: Option<String>,
    vars: std::collections::HashMap<String, String>,
}

Calling get_state() fails with an error:

Error in function set on type: bevy_mod_scripting_core::bindings::reference::ReflectReference: Value mismatch, expected: std::collections::HashMap<alloc::string::String, alloc::string::String, std::collections::hash_map::RandomState>, got: {foo: bar}.

An example/doc/test here would be great to show how one should pass maps between Lua and Bevy.

Specifically, docs for when to use ScriptValue or not would be useful. I haven't managed to fully understand this even from reading https://makspll.github.io/bevy_mod_scripting/generated_docs/types/scriptvalue.html?highlight=scriptvalue#scriptvalue

Edit: The general thing I'm trying to support here is sending a table of key/value pairs from the script to Bevy where the keys are only known at runtime.

[makspll]

At the moment hashmaps are supported as arguments and return values, but not as dynamic references, i.e. you cannot set them via reflection like this, maybe this error should be a bit better!

I can have a look to see if it's doable quickly.

In general you'd use ScriptValue as a catchall, think of it as mlua::Value, there's no need to use it unless you want to handle multiple different types of arguments

[makspll]

The PR linking to this should let you do that, HOWEVER, bevy still doesn't actually support indexing into maps via reflection :/