Typescript setup (second try)

[epicfaace]

First try was #1446. Doing this again because a lot of merge conflicts.

for file in src/components/*.js; do mv "$file" "${file%.js}.tsx"; done
for file in src/components/widgets/*.js; do mv "$file" "${file%.js}.tsx"; done
for file in src/components/fields/*.js; do mv "$file" "${file%.js}.tsx"; done
for file in tests/*.js; do mv "$file" "${file%.js}.tsx"; done
for file in playground/samples/*.js; do mv "$file" "${file%.js}.tsx"; done
mv playground/app.js playground/app.tsx
mv src/index.js src/index.tsx
mv src/withTheme.js src/withTheme.tsx
mv src/utils.js src/utils.tsx
mv src/types.js src/types.ts
mv src/validate.js src/validate.ts