feat(warn): Warn on colon shorthand usage on directive (fix #10191)

[afontcu]

fix #10191

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

Other information:

<template>
  <div v-if="something">foo</div>  <!-- does not throw a warning -->
  <div :v-if="something">foo</div> <!-- throws a warning -->
</template>

There are several things I decided that could be improved, so any advice or suggestion is greatly appreciated :)

• PR shows a warning for every bound attribute starting with v-. The reasoning is that "The v- prefix serves as a visual cue for identifying Vue-specific attributes in your templates." (source)
• Do we need more test cases?
• Is the warning message clear enough? If the attribute is not a valid Vue directive, should the message change?

Thanks!

[posva]

Looks good but now that I think about it, I think we could add the ="${value}" to the instead of part. Sorry I forgot 😆

[afontcu]

np! Actually I was about to add that bit too 😂 but after not seeing it in the suggestion I restrained myself. Updated!

[Vannsl]

Can't this PR be merged? :)