Add typescript as a valid parser value (#1318)

* Add typescript as a valid parser value

Now that typescript development has started, each contributor has been adding this as a supported flag themselves.

This just adds it as an allowed option, with a warning that support for this parser is experimental.

I also tried to structure this a bit future proof, in case other experimental parsers come along.

* Simplifies the code at vjeux's request

Rather than a notion of "experimental" parsers, we'll just leave typescript out of the documentation until it's supported.

bin/prettier.js

@@ -66,7 +66,7 @@ function getParserOption() {
     return "flow";
   }
 
-  if (value === "flow" || value === "babylon") {
+  if (value === "flow" || value === "babylon" || value === "typescript") {
     return value;
   }