Support multiple translation string tables

[AnmAtAnm]

Blockly currently only supports a single translation string table generated from messages.js and the msg/json files. This works well for core Blockly strings and the blocks we ship, but we don't have a path for external developers to include their own strings without modifying Blockly (and risking merge conflicts).

We need to design a means for developers to provide and load their own string tables, so that the strings can be referenced from custom blocks and mutators.

[Spoorthy1423]

@AnmAtAnm , i would like to work on this issue, please assign me

[BeksOmega]

This is actually quite a complicated issue that needs design work :/ so it's not open to contribution at the moment.