[embind] Allow Closure to compress registerType methods (#24610)

The reason 'fromWireType', 'toWireType' and 'readValueFromPointer' were
always protected from Closure is because we dynamically generate JS
functions, and property mangling would make it impossible for the
dynamically generated code to access properties declared statically.

However, we can easily work around that by binding corresponding methods
outside of the generated lambdas, and passing just the methods instead
of the whole type object.

Author: RReverser