chore: clean up gulpfile, use tsconfig.json.

[mprobst]

• Always use tsconfig.json as the one authoritative source for compilation.
• Specify noEmit so that IDEs don't generate files on disk.
• Override tsconfig.json options as needed:
  • noEmit: false to emit files
  • declaration: true to generate .d.ts files
  • typescript to specify the TS version used

[mprobst]

@alexeagle says LGTM.