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* generate type class strings for messages and services
* narrow down `TypeClass` type from `string` to known values
* generate typings for *Wrapper javascript classes
* generate constants as 'const enum'

These provides intellisense for typeclass names and allow constants to be referenced by value.

The constants are generated with '_Constants' suffix, since ros does not allow underscores in
the message name, this should be safe from any conflicts. This is also same as how the ros idl
generator exports the constants.

Author: Teo Koon Peng