fix(flex): fill styles

[kuzhelov]

Fixes #1348

This PR addresses the issue of expected behavior of <Flex fill />, and makes it aligned with the behavior suggested by spec:

fill - Orders container to fill all parent's space available.

BREAKING CHANGES

Effect of this fix is changed behavior of fill prop of Flex component in case if it is a child of other Flex.

<Flex>
  <Flex fill>
  </Flex>
</Flex>

If client would like to preserve this look, i.e. inner Flex to take only the minimal width necessary (and not the entire space provided by outer Flex), then

• fill prop should be removed for inner Flex
• provide vAlign=stretch prop to outer Flex

<Flex vAlign='stretch'>
  <Flex></Flex>
</Flex>

---

Description

From https://codesandbox.io/s/ulrbd?module=/example.js :

// outer
<Flex debug .. >
  <Flex fill .. >
     Inner Flex
  </Flex>  
</Flex>

Was

Now

[codecov]

Codecov Report

Merging #1352 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1352   +/-   ##
======================================
  Coverage    72.3%   72.3%           
======================================
  Files         764     764           
  Lines        5769    5769           
  Branches     1712    1712           
======================================
  Hits         4171    4171           
  Misses       1592    1592           
  Partials        6       6

Impacted Files	Coverage Δ
...eact/src/themes/base/components/Flex/flexStyles.ts	5.26% <ø> (ø)	⬆️

---

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[mnajdova]

Would suggest adding a screenshot test for catching possible regressions. Other than that looks good :)

[kuzhelov]

@mnajdova, just verified that there is no valid use case for the nested single Flex in Flex - case where, actually, issue was discovered. Would refrain from providing very contrived example in docs for now