refactor(Label): cleanup docs and simplify render

[dvdzkwsk]

This PR cleans up some inconsistencies in the doc strings, and simplifies the render method for readability. There are no functionality changes; this is simply a reduction of source code.

[dvdzkwsk]

Factories already handle falsy values in the first position, so we don't need to handle that ourselves.

[dvdzkwsk]

Remove reassignment of local variables. We can simply render elements inline in their target position.

[dvdzkwsk]

Make terminology consistent. Allowed prop values are "start" and "end", so we should use the same wording in the description.

[kuzhelov]

👍 👍

[kuzhelov]

UTs need to pass before merging

[kuzhelov]

UTs are fixed now - there was a problem with filtering logic implemented as part of shorthand tests

[dvdzkwsk]

Thanks for the UT fix, @kuzhelov.

[layershifter]

I really like changes in this PR, superior work 👍

---

But, there is an issue that causes visual regressions. Layout will add a gap if start/end are present, after these changes they will be always present and it will produce invalid gaps 😭

We have already discussed this issue with @kuzhelov, we can make following changes:

const startImage = imagePosition === 'start' && imageElement
const starIcon = iconPosition === 'start' && iconElement

const hasStart = startImage || startIcon

<Layout start={hasStart && (
  <>
    {startImage}
    {startIcon}
  </>
)}

It can be a solution, but I'm open for better ideas.