core: rename Element to LHElement

[connorjclark]

While checking out the ts 3.7 beta, this typing mistake became apparent. We use the global Element type declaration (dom lib) where we really want our lib/element.js. Renaming to LHElement is a simple solution to this name clash.

the error was uncovered likely thanks to changes required for this bugfix: https://devblogs.microsoft.com/typescript/announcing-typescript-3-7-beta/ "Local and Imported Type Declarations Now Conflict"

[patrickhulce]

LGTM

I'd say we could be more descriptive like DriverProxyElement or just ProxyElement something since I had to stare at this for a bit to remember why we have something called Element at all, but not required :)