feat: add test for construct using unit struct

[mgi388]

Pushing this PR up so we can document how to construct unit structs. The first commit of this PR will fail with:

---- script_test - lua - /construct/construct_unit_struct.lua ----
error: Error in function construct : Error converting argument 2: Value mismatch, expected: std::collections::HashMap<alloc::string::String, bevy_mod_scripting_core::bindings::script_value::ScriptValue, std::collections::hash_map::RandomState>, got: [].
Context:
stack traceback:
        [C]: in function 'construct'
        [string "crates/languages/bevy_mod_scripting_lua/src/l..."]:2: in main chunk

Not sure how to get this to work without passing dummy values like this:

local type = world.get_type_by_name("UnitStruct")
local constructed = construct(type, {
    dummy = true
})

assert(constructed ~= nil, "Value was not constructed")

@makspll feel free to push to my PR if you have a fix. Hopefully this test documents this for future users.

[makspll]

Ah you've hit an interesting case!
At the moment ScriptValue has both a Vec and a Map variant,
Now during conversion from lua, mlua has to know the target type to convert to, the way it currently does that, is by looking at the type of keys in your table.

Obviously lua doesn't actually have a distinction between maps and lists, which is the root cause here!

One way to deal with this would be to introduce a map({..}) constructor, which explicitly takes in either a Vec or a HashMap.
Another way would be to introduce a new FromScript implementing primitive ListOrMap, I guess you can already do this with Union, and use that instead of Map or List everywhere, but I think the map constructor way is less restrictive on binding authors

[makspll]

Once #329 merges this should work

[mgi388]

Nice! Does the PR need a Rhai test too?

[makspll]

Yes that would be appreciated