v0.6.4

Added

• TypeScript errors are now displayed in the editor with red underlines. A
  tooltip displaying the error is shown on hover.

  • Note that only basic/syntactic errors are currently shown, because typings
    of dependencies are not currently available to compilation.

• Added CSS property --playground-error-border, the border-bottom of code
  spans with an error (2px red dashed by default).

• Added CSS shadow part diagnostic-tooltip for styling the tooltip that
  appears when hovering over an error.