bug: use singleThread for Bun

[erhant]

here: https://github.com/erhant/circomkit/blob/main/src/core/index.ts#L305 we can add an additional argument that provides { singleThread: true } when we are using Bun.

Bun can be detected with:

export function isBun(): boolean {
  // ignore is required as `Bun` is not defined in Node
  // @ts-ignore
  return typeof Bun !== "undefined";
}