fix(codegen): support IE11 and Edge use of "Esc" key (close #7880)

[matheusazzi]

Closes #7880

IE11 and Edge use "Esc" as key name, other browsers use "Escape".

I was able to test on Edge 16/15/14 and IE11.

• More reference: https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/5290772/
• Reproduction link: http://codepen.io/anon/pen/QjxPGB

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

• Bugfix
• Feature
• 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 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

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)