Set default "jsx" mode to react

[hasparus]

Checklist:

• There is an associated issue that is labeled
  'Bug' or 'help wanted' or is in the Community milestone
• Code is up-to-date with the master branch
• You've signed the CLA
• Ask if this is actually needed
• Run jake runtests locally -- they fail.
• Fix failing unit tests
• Update default value in the docs -- currently it's "Preserve"
  • https://www.typescriptlang.org/docs/handbook/compiler-options.html
• Follow documentation section in contributing wiki.
  • This PR is a breaking change for people that ignore the error and depend on current default behavior.

Description

If compilerOptions.jsx is undefined we treat it like if it were JSXEmit.React.

I don't feel like this is the right and the most elegant solution and I didn't fix tests yet so this is WIP.
I feel I need some more stepping around the code to find a place to actually set default config values.

Compiler still logs error when JSX flag is not explicitly set.
IMO is not too pretty so I have a fix for it in separate PR (hasparus#1) I'd like to merge into this branch.

Solves #8529

[msftclas]

All CLA requirements met.

[RyanCavanaugh]

Looks like this work in progress didn't quite finish - let us know (maybe in the linked issue) if we can help out with pointing you at similar PRs. Closing for now - thanks!