feat(store): bind mutation and action handlers to store

[pi0]

• Feature
• Not affecting current API

As of vue's runInNewContext enhancement for SSR, everything should be stateless in server side to prevent each request affecting others so when implementing Actions which need access to ssrContext (Like axios with configured proper auth headers - nuxt-community/legacy-modules#26) the only way is passing those references from caller as action/mutation parameters (example), this is working but makes usage ugly and really harder. This enhancement is required for workaround and fixes by simply binding handlers to store instance. So we can inject state into store and access it using this inside action/mutation functions. (this is currently undefined inside handlers)

[LinusBorg]

@pi0 It's currently quite common to write actions as fat arrow functions. those would have to be converted to normal functions, right?

[pi0]

@LinusBorg Yep, actually using bind to context instead of using 3rd argument or something like that was intentional as arrow functions are commonly considered pure functions without any side effects like access and modify store instance. Another point with this context binding is having a familiar API just like vm instances which we can access things using this.*.