[scripting] Scripting introruction

[jkb0o]

This is scripting work in progress.

• see asset-scripting example:

 cargo run --example asset-connections

• EmlAsset now holds handle into EmlRoot
• EmlRoot consist of root node and script: Option<ScriptingDeclaration>
• EmlAsset::write(world) now parses rhai script if ScriptingDeclaration provided and put ast tree into Scripts resource (HashMap<Handle, Ast>). This way any EmlAsset could have associated script via Handle<EmlAsset>