Enable the use of TypeScript in this repo

[bradenmacdonald]

We're seeing a lot of defaultProps deprecation warnings in the JS console when running MFEs.

Soon we can use the nice react-prop-types-typescript codemod to convert all our propTypes and defaultProps to TypeScript/JS defaults. But before we can do that we need to support TypeScript and convert the .jsx files to .tsx.

This PR:

• Enables TypeScript support
• Adds type stubs for frontend-platform which still doesn't have types; haven't fixed upstream because frontend-base was supposed to do this.
• Converts everything in src/studio-header from JSX to TSX
• Fixes a few minor type warnings that surfaced

This is only a build/test PR; it should not affect the actual runtime code itself, except for one line that I've called out in a comment.

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[bradenmacdonald]

This is the only change that could have a runtime effect. Everything else is just test code / type checking.

But TypeScript correctly pointed out that we weren't passing the required isMobile here. I assume it's correct to just pass it in, but ignoring the real value and passing isMobile: false would be more consistent with how the code worked before.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.56%. Comparing base (3be690b) to head (9025974).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
+ Coverage   68.75%   72.56%   +3.81%     
==========================================
  Files          48       49       +1     
  Lines         432      492      +60     
  Branches       97      100       +3     
==========================================
+ Hits          297      357      +60     
  Misses        132      132              
  Partials        3        3              

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[bradenmacdonald]

@brian-smith-tcril Would you be able to take a look at this?