BREAKING(Text): change font size scheme and Text size API

[codepretty]

Before

<Text size="xs" />
<Text size="sm" />
<Text size="md" />
<Text size="lg" />
<Text size="xl" />
<Text size="2x" />
<Text size="3x" />
<Text size="4x" />

After

<Text size="smallest" />
<Text size="smaller" />
<Text size="small" />
<Text size="medium" />
<Text size="large" />
<Text size="larger" />
<Text size="largest" />

---

• Updated sizes to be smallest, smaller, small, medium, large, larger, largest
• Updated doc examples to show font sizes that are supported by the theme

Was

• showed hard-coded font sizes that were the same for all themes
  

Now

• shows only the font sizes that are defined in a theme's siteVariables
  

[miroslavstastny]

By this change, you are defining a contract for all themes that they must define their sizes in fontSizes.
How do you plan to enforce it? It should be at least added to ISiteVariables

[miroslavstastny]

I would also add the size to the content so I can immediately see the size names in docsite.

[codecov]

Codecov Report

Merging #214 into master will decrease coverage by 0.07%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
- Coverage   91.45%   91.37%   -0.08%     
==========================================
  Files          63       63              
  Lines        1158     1160       +2     
  Branches      153      176      +23     
==========================================
+ Hits         1059     1060       +1     
- Misses         95       96       +1     
  Partials        4        4

Impacted Files	Coverage Δ
src/components/Text/Text.tsx	100% <ø> (ø)	⬆️
src/components/Chat/ChatMessage.tsx	93.61% <ø> (ø)	⬆️
src/components/Provider/Provider.tsx	60% <33.33%> (-0.47%)	⬇️

---

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[levithomason]

We will consider how to enforce this with typings later. This topic also warrants an RFC on /specifications for how we deal with font sizes and component sizes at the framework API level. The changes here are the result of many conversations and ideas with Fabric, teams around MS, and the open source. We chose a middle ground path for sizes for now. We are open to future changes.