This commit adds new option to "Worker" Web API that allows to
configure permissions.
New "Worker.deno.permissions" option can be used to define limited
permissions to the worker thread by either:
- inherit set of parent thread permissions
- use limited subset of parent thread permissions
- revoke all permissions (full sandbox)
In order to achieve this functionality "CliModuleLoader"
was modified to accept "initial permissions", which are used
for top module loading (ie. uses parent thread permission set
to load top level module of a worker).
* establish basic event loop for workers
* make "self.close()" inside worker
* remove "runWorkerMessageLoop() - instead manually call global function
in Rust when message arrives. This is done in preparation for structured clone
* refactor "WorkerChannel" and use distinct structs for internal
and external channels; "WorkerChannelsInternal" and "WorkerHandle"
* move "State.worker_channels_internal" to "Worker.internal_channels"
* add "WorkerEvent" enum for child->host communication;
currently "Message(Buf)" and "Error(ErrBox)" variants are supported
* add tests for nested workers
* add tests for worker throwing error on startup
