Extend config keyCodes property type to accept an array of numbers

[alexeden]

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

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

Modification of Vue's static config property type definition.

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

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), 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)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

Other information:

Per the documentation of the Vue config's keyCodes object, the value of a keyCodes property may be either a number or an array of numbers. The current keyCodes type definition only accepts a single, non-array number. This PR corrects the type definition to reflect both types of acceptable values.