refactor: Add isString

[243083df]

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

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

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

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• [x] New/updated tests are included

[dsonet]

Is this really needed or something just more like over-engineering?

[lusarz]

For me it's good approach to create isString like methods. It increases readability of code.

There are methods like isTrue, isFalse, isObject in shared/util, so why don't create similar for isString ?

Please also take a look here: https://github.com/vuejs/vue/blob/f7b5c86ef2620c1d7ec275f86ae0f380ccaea4ef/src/platforms/web/util/class.js#L37-L39 Mixing different levels of abstraction (isObject near typeof value === 'string') is not a good practice.

[243083df]

As said lusarz, there is methods like isTrue|isFalse|isObject|isPrimitive, so i dont see over-engineering here