Allow custom versions of blockly_compressed.js to be used

[RoboErikG]

Our CodeGeneratorService uses a specific version of blockly_compressed.js. We should allow a developer to pass in a custom one in case they've modified the core generation code or made other modifications.

[RoboErikG]

Also blocks_compressed and the messages file (en.js)

[AnmAtAnm]

This seems like a narrow use case that is easily solved by the end developer replacing a couple of well defined files. Do we really need to do anything for this, other than document where those files are?

[RoboErikG]

Yeah, you're right. Let's just document those files.

We can also rename javascript_compressed.js to -> generator_core_compressed.js and tell devs to replace it with the core language generator of their choice.