feat(app/ts): add Vuex typings, fix #8511, #8500

[IlCallo]

Closes #8511, #8500

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch and not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on a Electron app
• Any necessary documentation has been added or updated in the docs (for faster update click on "Suggest an edit on GitHub" at bottom of page) or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Static import of 'storeKey' wouldn't work, so I had to go for a dynamic import.
The key won't be present into JS codebases as it's not needed

I'm not sure if it's really needed on server-entry.js too, but better safe than sorry

I also fixed a docs code example which I didn't update during Qv2 migration

[francoism90]

This does seems to fix #8500 as well. :)

[francoism90]

@IlCallo I did found out you need to give a Symbol description. See https://eslint.org/docs/rules/symbol-description

[IlCallo]

@francoism90 thanks for taking the time to try this out!