Implement formal support for translation of messages in plugins

[moniika]

Category
Plugins

Component

n/a

Is your feature request related to a problem? Please describe.

Plugins that have translatable text on UI element require translation support, but there is no "official" way to support translation in plugins.

Describe the solution you'd like
google/blockly#1169

Describe alternatives you've considered

Continue using the same approach as in block-plus-minus and workspace-backpack plugins:
https://github.com/google/blockly-samples/blob/fb16a806ce44aeb5296647f77bbc5813c3b5ed93/plugins/workspace-backpack/README.md#blockly-languages
https://github.com/google/blockly-samples/blob/fb16a806ce44aeb5296647f77bbc5813c3b5ed93/plugins/block-plus-minus/README.md#blockly-languages

Additional context
The design for this will probably require core changes and existing plugins with english strings should be updated to use the new system.

[alschmiedt]

Additional info:

In typed modal we addressed this by allowing developers to call setLocale with a messages object. See 'Blockly Languages` on the npm page.