ADD addon-contexts: Preact support

[leoyli]

Issue: #6659

What I did

• [feature] Add Support to Preact.
• [doc] Update addon support list and example stories into Preact-kitchen-sink.
• [doc] Update README.md file for redirect the reader on the framework support table.

Note, currently Preact only support the old context API. The new createContext API is just released in Preact X beta.0; therefore, I add a TODO and comment out the Language example story for Preact (to avoid foreseeable to-be-out-dated works).

How to test

Live Example:
https://monorepo-git-add-addon-contextspreact-support.storybook.now.sh/examples/preact-kitchen-sink/?path=/story/addons-contexts--simple-css-theming&contexts=CSS%20Themes=Forest

[vercel]

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Latest deployment for this branch: https://monorepo-git-add-addon-contextspreact-support.storybook.now.sh

[codecov]

Codecov Report

Merging #6660 into next will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##            next    #6660      +/-   ##
=========================================
- Coverage   40.4%   40.38%   -0.03%     
=========================================
  Files        637      639       +2     
  Lines       8778     8783       +5     
  Branches     643      643              
=========================================
  Hits        3547     3547              
- Misses      5133     5138       +5     
  Partials      98       98

Impacted Files	Coverage Δ
addons/contexts/src/preview/frameworks/preact.ts	0% <0%> (ø)
...ddons/contexts/src/shared/@mock-types/_preact.d.ts	0% <0%> (ø)

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[codecov]

Codecov Report

Merging #6660 into next will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##            next    #6660      +/-   ##
=========================================
- Coverage   40.4%   40.38%   -0.03%     
=========================================
  Files        637      639       +2     
  Lines       8778     8783       +5     
  Branches     643      643              
=========================================
  Hits        3547     3547              
- Misses      5133     5138       +5     
  Partials      98       98

Impacted Files	Coverage Δ
addons/contexts/src/preview/frameworks/preact.ts	0% <0%> (ø)
...ddons/contexts/src/shared/@mock-types/_preact.d.ts	0% <0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 55d3878...1ae1e5e. Read the comment docs.

[shilman]

Great work @leoyli !! 🎉