Add Support for design-time decorators

Now that decorators are supported in TypeScript (#2249), consider adding support for ambient/design-time-only decorators:

Ambient decorators can be an extensible way to declare properties on or associate special behavior to declarations; design time tools can leverage these associations to produce errors or produce documentation. For example:

Use cases:

• Deprecated/Obsolete, to support warning on use of specific API’s:

interface JQuery {  
    /**  
     * A selector representing selector passed to jQuery(), if any, when creating the original set.  
     * version deprecated: 1.7, removed: 1.9  
     */  
    @@deprecated("Property is only maintained to the extent needed for supporting .live() in the jQuery Migrate plugin. It may be removed without notice in a future version.", false)  
    selector: string;  
}

• Suppress linter warning:

@@suppressWarning("disallow-leading-underscore")   
function __init() {  
}

Proposal

Design-time (Ambient) decorators are:

• ambient functions
• have a special name starting with "@"
• are not emitted in output JS code, but persisted in .d.ts outputs.

Application

• Applying a design-time can only accept constant values. Variables would not be observable by the compiler at compile time. Here is the set of possible values:
  • string literal,
  • number literal,
  • regexp literal,
  • true keyword,
  • false keyword,
  • null keyword,
  • undefined symbol,
  • const enum members,
  • array literals of one of the previous kinds,
  • object literal with only properties with values of one of the previous kinds

Start by reading this proposal alongside TypeScript issue #2249 and the existing decorator support. Determine the design for ambient functions, constant-only arguments, and .d.ts preservation; the work is done when design-time decorators are accepted, omitted from JavaScript output, and persisted in declaration output.