Add prettier user test and fix associated crash

[sandersn]

While adding prettier to the user tests, I uncovered another assert in getJSDocHost when looking up type references in a @property tag inside a @typedef tag. This PR contains the fix as well as adding prettier to the user tests.

Crash

• getJSDocHost didn't work correctly when called on a @property tag.

Incorrect error

• Can't import package.json -- Resolve the modue names with "modulename.json" to json files when doing node module resolution #22167
• Can't import npm package that exports json instead of js -- Add html-tag-names DefinitelyTyped/DefinitelyTyped#25335
• node-ignore has wrong types -- Fix type of filter kaelzhang/node-ignore#36
• chalk has wrong export structure -- fix: use export = for typescript module definition chalk/chalk#256
• Can't refer to typedef from another file -- In JS, typedefs should be automatically exported #23692

Annoying

• Can't pass null to optional parameter with strictNullChecks.
• Can't add instance properties to Error instances
• [] is still often never[] with strictNullChecks (eg as a property of an object literal, or as the initial value of a reduce)
• When a mutable variable is initialised with a specific member of a discriminated union, we need a type annotation in order to specify that the variable's type should actually be the entire union.
• Browser detection code needs tweaks to be well-typed, even though nobody cares.
• Auto types don't scale well to 1,000-line functions, and type declaration is blocked by the typedef reference feature.
• Have to cast partial objects that are lazily initialised.

Improved hygiene

• strictNullChecks forces some checks for undefined that are not obviously impossible.
• strictNullChecks forces a few casts for undefineds that are obviously impossible (eg from queue.pop just after checking queue.length).
• @param {number?} n doesn't make an optional parameter. @param {number} [n] and @param {number=} n do.
• Too many arguments passed to a few different functions.
• [].concat(x,y,z) has to be changed to [...x, y, ...z]

Found possible bug

• Forgot to check result of String.match for null.
• glimmer requires ASTPlugins to have a name property, but parser-postcss forgot to provide one.

Discussion

As you can see, there were lots of problems adding checkJs to prettier. None of them were particularly difficult to resolve, but there were a lot of small tasks that added up. Without strict null checks, there was only one bug and two hygiene items. Compare that to the whole pile of annoyances and incorrect errors.

[mhegazy]

Auto types don't scale well to 1,000-line functions, and type declaration is blocked by the typedef reference feature.

what do you mean by not scaling?

[sandersn]

The chance of doing something that confuses an auto type in a 1,000-line function approaches 1. In prettier, assignments plus each+lambda iteration confuse it.

If you had 100 10-line functions, each with an auto type, the number of constructs we don't handle would not be much higher, but you'd have many auto-typed variables with no problems.