static type checking (js + jsdoc)

[art-in]

From v2.3 (27.04.17) typescript now supports type checking .js files.
And it already can gather type definitions from jsdoc annotations.

https://github.com/Microsoft/TypeScript/wiki/Type-Checking-JavaScript-Files
https://github.com/Microsoft/TypeScript/wiki/JSDoc-support-in-JavaScript

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replace runtime type/param checks with static checks:

• src/
• test/
• (params) typeof/instanceof param checks
• (params) required-params (forked to #65)
• (params) PropTypes (with @typedef Props) (forked to #66)
• guard-object-props
• webpack --watch mode (gulp build:watch, test:watch)
• webpack normal mode (gulp build, test)
• add separate gulp task to check types (test:static:types)
• code editor highlights
• no unnecessary @ts-ignore / @ts-nocheck
• gotchas.static-typing.md

[art-in]

added gotchas/typescript.md to track history of typescript usage

summary:

• typescript has bugs and unsupported syntax features (not much, but little pain)
• typescript allows to type cover surprisingly complex cases (eg. dynamic generics in connect.d.ts)
• jsdoc support in typescript is bad (pain)
• complex types would be hard or impossible to define in jsdoc (need .d.ts anyway)
• due to lack of jsdoc support, writing typescript directly - now looks less bad choice than before
• majority of important code places is type covered, but there is still a lot of implicit any's (needs work)
• static checking helped to find 2 hidden bugs that were hard to find in run-time
• adding static type checking for existing project is pain at start, but worth it in long-run