Add guaranteed one time setup

[makspll]

Changes APIprovider interface to:

/// Implementors can modify a script context in order to enable
/// API access. ScriptHosts call `attach_api` when creating scripts
pub trait APIProvider: 'static + Send + Sync {
    /// the type of script engine/context the API is attached to, this must be the same as the APITarget of the ScriptHost meant to receive it.
    type APITarget: Send + Sync + 'static;
    /// The type of script context the APIProvider works with, must be the same as the ScriptContext of the target ScriptHost.
    type ScriptContext;
    /// The type of documentation fragment produced by the APIProvider, must be the same as the DocTarget of the target ScriptHost.
    type DocTarget: DocFragment;
    /// The type of script asset, must be the same as the ScriptAsset of the target ScriptHost
    // type ScriptAsset : CodeAsset;
    
    /// provide the given script context with the API permamently.
    /// Depending on the host, API's may be attached on a per-script basis
    /// or on a per-engine basis. Rhai for example allows you to decouple the State of each script from the
    /// engine. For one-time setup use `Self::setup_script` 
    fn attach_api(&mut self, ctx: &mut Self::APITarget) -> Result<(), ScriptError>;

    /// Setup meant to be executed once for every single script. Use this if you need to consistently setup scripts.
    /// For API's use `Self::attach_api` instead.
    fn setup_script(&mut self, script_data: &ScriptData,ctx: &mut Self::ScriptContext) -> Result<(), ScriptError>;

    fn get_doc_fragment(&self) -> Option<Self::DocTarget> {
        None
    }
}

Means that script hosts can setup the API once (as rhai can) but also provide one time setup for scripts depending on some script properties