Implement externs generation for transpilers

[vlaaad]

Task (REQUIRED):
Initial support for transpilers does not include externs generation for built-in Defold Lua APIs. This limits their usefulness to standalone Lua modules for e.g. game logic. To support interop with Defold APIs, the transpilers need to know about APIs defined in the project, sourced from the engine (i.e. sdoc files) and script_api files from extensions. We should design and implement this support.

Expected outcome (REQUIRED):
Transpiler extensions know about Defold APIs.