This removes the EventListener, EventInit and CustomEventInit constructors from the userland globals. The type exports stay.
I removed the internal classes as well. EventListener's implementation seemed to be doing some bookkeeping on handled events but that's not being used anywhere so I assume it's old debug stuff. The other two are completely redundant.
Instead of using core/snapshot_creator.rs, instead two crates are
introduced which allow building the snapshot during build.rs.
Rollup is removed and replaced with our own bundler. This removes
the Node build dependency. Modules in //js now use Deno-style imports
with file extensions, rather than Node style extensionless imports.
This improves incremental build time when changes are made to //js files
by about 40 seconds.
