Typing changes for VariablesState and Story

[evanwalsh]

Checklist

• [x] The new code additions passed the tests (npm test).
• [x] The linter ran and found no issues (npm run-script lint).

Description

This makes the experience of using inkjs from TypeScript slightly better by:

1. Adding typed access to a story's VariablesState. Now, a TypeScript user can do story.variablesState['name'] or story.variablesState.$<string>('name') and either get the convenience that the indexed access affords or the typed access via the generic param of $. Also, the $ function didn't allow for fetching a value without a TypeScript error. That's fixed!

2. In Story.BindExternalFunction, the lookaheadSafe parameter is now false by default. According to Ink's source:

   Usually, you want to pass 'false', especially if you want some action to be performed in game code when this function is called

   As far as I can tell from the C#, false should generally be the default. This makes the specs' usage of BindExternalFunction correct and saves TypeScript users from having to pass true or false everytime!

[y-lohse]

Hello, thank you for all 3 PRs! I've had a quick look already and they all seem interesting, I will try my best to do a review as soon as possible… but it might be a few days :)

[smwhr]

Next release :

• Behaviour of variablesState has been modified in latest.
• Added default true to lookaheadSafe