Esm eslint

[brettz9]

Builds on #33.

• Breaking change: Enhancement: Add native ESM distribution, with exports property in package.json
• Refactoring: Add Rollup/Babel/Terser
• Build: Have generator file produce consumable source files directly rather than logging to console
• Linting: Switch from JSHint to ESLint (using rules, e.g., indent and prefer-const, in place on other estools project)
• Testing: Convert coffeescript test files to ES6
• Testing: Add nyc
• Testing: Check unmatched high surrogates and full coverage for AST
  expressions (further true isExpression and isStatement's and
  false isProblematicIfStatement), bringing to 100% coverage
• Travis: Drop previous versions for 12, 14, 16