Question: value type check functions

**Is your feature request related to a problem? Please describe.**

There are lots of type check functions in std (`isBoolean()`, `isObject()`, `isString()` etc). As of now, each module implements its own functions (`isObject()` is implemented in `assert/object_match.ts`, `expect/_utils.ts`, `yaml/utils.ts` for example).
I wonder if we should gather them in `@std/internal` or even make them public in another mod.

**Describe the solution you'd like**

Put all type check functions at the same place which all std mods import them.

**Describe alternatives you've considered**

Leave as is. There is the danger that once we have some type check functions, then we should provide all of them and over-engineer simple checks. `lodash` provides lots of checks which might not be necessary.