Credit syntax definition and theme authors with new --acknowledgements option (#1971)

The text that is printed is generated when building assets, by analyzing LICENSE
and NOTICE files that comes with syntaxes and themes.

We take this opportunity to also add a NOTICE file as defined by Apache License 2.0.

assets/create.sh

@@ -53,7 +53,7 @@ bat cache --clear
     done
 )
 
-bat cache --build --blank --source="$ASSET_DIR" --target="$ASSET_DIR"
+bat cache --build --blank --acknowledgements --source="$ASSET_DIR" --target="$ASSET_DIR"
 
 (
     cd "$ASSET_DIR"