Improve the JSX situation

[max-kamps]

Currently, the following problems exist:

• util.jsxCreateElement is slow
  • Check if document.createElement is the culprit, maybe switch to string templating
  • Investigate writing an alternate emitter plugin that outputs more optimized and inlined code
• jsx.d.ts is morally wrong
  • Automatically generate it from some machine-readable data format that contains all valid HTML tags and attributes
  • Investigate typechecking children correctly (not all HTML elements can contain all other HTML elements)
  • Investigate new TS 5.1 type checking features (see issue #47)

[max-kamps]

The changes in TS 5.1 ended up not being helpful. Neither will 5.2. There is an active TypeScript issue about a feature that would allow correctly typechecking children of JSX elements: https://github.com/microsoft/TypeScript/issues/14729