feat(compiler): add warning when directives other than v-for and v-if used on template tag

[superMDguy]

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)

Other information:
According to the docs, the v-show directive doesn't support the <template> element. So, if a v-show is used on a template, it is essentially ignored. However, it currently does that without any warning. This pull request adds a warning when a v-show directive is used on a template tag.

[yyx990803]

Hey, thanks for the PR! A few things to note:

1. Actually, the only directives that make sense on <template> are v-if and v-for. So we can check and warn not only v-show, but any directives that are not v-if or v-for.

2. This may not be the best place to have a big chunk of logic since it kinda breaks the readability of the function which is mostly branches. We should extract it into a separate function, something like:

if (process.env.NODE_ENV !== 'production') {
  checkInvalidTemplateDirs(el)
}

[superMDguy]

I've made the changes. I was thinking other directives also wouldn't be allowed, but I wasn't sure if that was the case 100% of the time.

[felixbuenemann]

Shouldn't v-slot also be allowed?