Allow inline type predicates

From #5731

if (<foo is Element>(foo.nodeType === 1)) { 
    // Assume foo is Element here
}

A proposal from @sandersn: https://github.com/Microsoft/TypeScript/issues/5731#issuecomment-162586789

Type Predicate Expressions

A type predicate expression allows you to narrow a union type with a single expression. The syntax is

<variable is type> boolean-expression

When the expression is used in a conditional context, the true branch of the conditional narrows variable to type, while the false branch narrows variable by removing type.

The type predicate expression is of type Boolean, with apparent type of type predicate. The right hand side must be an expression of type Boolean. The left hand side's variable must be a name bound in the current scope. The left hand side's type must be a name bound in the current scope.

Open questions

1. How does variable capture work? Can a type predicate expression be returned from a function and used to narrow a variable that's no longer in scope? This could be defined to cause an error, but that restriction is neither obvious nor easy to use.
2. Why reuse the type assertion syntax? The type of the right expression is checked to be Boolean, unlike assertions, and the resulting type is still Boolean, also unlike assertions.

先阅读从 issue #5731 链接的提案以及此处的 Type Predicate Expressions 部分。在继续之前，解决关于变量捕获、语法和 Boolean 检查的所列问题；当 inline predicate 语法及其在 true 和 false 分支中的 narrowing 行为已针对该示例完成规范定义并得到支持时，即视为完成。