feat(parser): throw error when using interpolation instead of expresssion (#9038)

[kamilchlebek]

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

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)

Yes it's already an open issue with good first issue label. Resolves: #9038

[yyx990803]

Why dirRE.test here? Seems unnecessary.

[kamilchlebek]

I've added this to keep the same behavior as in place where previous warning exists (line 596). As you can see that previous warning is shown only when dirRE.test is false.

processSlot is executed outside of processAttrs thus I've added this additional check, to prevent double warnings