This is major refactoring of the structure of the core codebase.

There are no changes to any existing test cases.

Summary of changes:

• Drop Node 0.12 support, now >=4 to align with eslint/eslint
• Remove now redundant object-assign dependency, and replace its usage with native Object.assign
• Remove features.js and token-translator.js - they were never used
• Use same lint configuration as eslint/eslint
• Upgrade all dependencies to latest
• Update ast-node-types.js to include all known converted ESTreeNode types
• Add errorOnUnknownASTType parser option to throw if a generated ESTreeNode type is not found in ast-node-types.js (this is disabled by default, but enabled in tests)
• Separate utility functions out into a new file, node-utils.js, to make the codebase much easier to reason about, especially w.r.t. which helpers mutate the current node, and which are pure functions
• Move all ts.* helper functions into abstractions within node-utils.js so that it is clear what our dependencies on TypeScript actually are
• Add brand new general utilities such as findFirstMatchingChild and findFirstMatchingAncestor which take in predicate functions, and are built upon by other utility functions. These also allow me to remove some ts.* dependencies from the codebase
• Move core convert() function into its own file, convert.js, and increase what is explicitly passed into it rather than just closing over all outer scopes. Again, this greatly helps with clarity and focused responsibility
• Add block scoping to more complex switch cases in convert.js, taking advantage of let and const for variable declarations. No more battling to come up with unique variable names within a 1000+ line function!