fix(vue-app): check plugin key on Vue.prototype instead of Vue

[simplesmiler]

Fix for an obscure bug that got introduced in e8aca9e.
Triggered when you inject something under a key, that was already used by other library.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Example (using https://github.com/stalniy/casl/tree/master/packages/casl-vue):

// plugins/casl.js
import Vue from 'vue';
import { Ability, abilitiesPlugin } from '@casl/vue';
export default function(context, inject) {
   const ability = new Ability(...);
   Vue.use(abilitiesPlugin, { ability }); // defines $ability and $can on Vue.prototype
   inject('ability', ability); // error: Can not redefine '$ability'
}

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[simplesmiler]

Also, it appears that a proper reentrance check is done above via installKey. Because of this, maybe we should warn developer instead of silently skipping?

[codecov-commenter]

Codecov Report

Merging #7656 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #7656   +/-   ##
=======================================
  Coverage   70.18%   70.18%           
=======================================
  Files          88       88           
  Lines        3756     3756           
  Branches     1020     1020           
=======================================
  Hits         2636     2636           
  Misses        911      911           
  Partials      209      209           

Flag	Coverage Δ
#unittests	70.18% <ø> (ø)

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