Consolidate `createLocalVariables` functions into one

[six7]

Having two different files called updateVariables is a little confusing.

controller.ts -> asyncHandlers.updateVariables – this is a different function to updateVariables which is called in createLocalVariablesInPlugin.ts. controller.ts -> asyncHandlers.createLocalVariables asyncMessageHandlers/createLocalVariables.ts -> createLocalVariablesInPlugin(...) createLocalVariablesInPlugin.ts -> updateVariables

In a future refactor it could be worth reducing the number of functions re message handling and reuse more code. Moving between many different files/functions makes it harder to track the data flow.

Originally posted by @macintoshhelper in https://github.com/tokens-studio/figma-plugin/pull/2568#discussion_r1611720464