Transform JSX spread children

[Jack Bates (jablko)]

#11591 added support for parsing and emitting JSX spread children, however transformers/jsx.ts ignores JsxExpression#dotDotDotToken, so roughly it transforms <div>{...todos}</div> -> React.createElement("div", null, todos) vs. React.createElement("div", null, ...todos).

This isn't a problem for React.createElement() because that runtime treats both cases equivalently (it flattens children), however it is an issue for pickier runtimes.

This PR adds factory.createSpreadElement() to transformers/jsx.ts if JsxExpression#dotDotDotToken is defined.

[TypeScript Bot (typescript-bot)]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Wesley Wigham (weswigham)]

A new test that doesn't downlevel all the way to the __spreadArray helper (so targeting es6) would be nice, just to capture the difference simply. Other than that, this looks good.

[Jack Bates (jablko)]

Wesley Wigham (@weswigham) Thanks! Assuming that downleveling and JSX are independent, can we add @target: es6 to the existing test, to clarify the output? Nathan Shively-Sanders (@sandersn) mentioned here that tsxSpreadChildrenInvalidType.tsx covers the jsx: react emit.

That's what I've done in the added commit. Otherwise I'd copy tests/cases/conformance/jsx/tsxSpreadChildren.tsx -> tsxSpreadChildren_es6.tsx and add:

// @jsx: react
// @target: es6

Not sure if retaining both targets (default and es6) adds anything though?