Add toStringTag value on JSG resource types

[garrettgu10]

This PR ensures that Object.prototype.toString() returns the correct values for built-in objects (i.e. ResourceTypes).

This means that for instance new Headers().toString() will output [object Headers] instead of [object Object].

[dom96]

what's the difference between getMemoryName and getName?

[jasnell]

what's the difference bretween getMemoryName and getName

jsgGetMemoryName(...) is used while generating the heap snapshot. While for jsg::Object instances its value is currently the same name as the class, it does not need to be and could change in the future. We should not rely on its value for stable user-facing API.