export ThreadManager to allow auto-generation of TypeScript types

[aspiers]

threadman.js exports the buildThreadManager() function which returns an instance of the ThreadManager class.

However the ThreadManager class itself is not exported. This can be problematic; for instance, when auto-generating TypeScript declaration files from the source via tsc, it results in the following error:

src/engine.js:1:1 - error TS9006: Declaration emit for this file requires using private name 'ThreadManager' from module '"src/threadman"'. An explicit type annotation may unblock declaration emit.

1 import WasmField1 from "./wasm_field1.js";
  ~~~~~~

This is simply solved by exporting ThreadManager.