use namespaced metadata for targets

As briefly discussed in Slack, maybe it makes sense to refactor all the target-specific metadata (`@:jsRequire`/`@:cppInclude`/etc.) into namespaced variants (`@:js.require`, `@:cpp.include`). If we go with that, we can slowly deprecate the old ones, making them hidden aliases for the new ones and eventually emitting a deprecation warning.

One question here is what to do with target-specific metadata that is shared between a couple different targets, like `@:expose` for JS and Lua, or `@:functionCode` for C++, C# and Java.