Adding experimental online parser (#2770)

* Adding experimental online parser

* Improve Typings

* Update grammar structure

* Move rules to js

* Fix build

Co-authored-by: Ivan Goncharov <ivan.goncharov.ua@gmail.com>

src/language/experimentalOnlineParser/README.md

@@ -0,0 +1,26 @@
+## Experimental Online Parser
+
+This directory contains an experimental online parser based on JSON rules set. It is a state-full parser which parses a source string incrementally i.e. emits a token each time.
+
+The parser is being migrated from graphiql to graphql-js and may have frequent breaking changes.
+
+Example:
+
+```js
+import { OnlineParser } from 'graphql/language/experimentalOnlineParser';
+
+const source = `
+  query SomeQuery {
+    some_field {
+      another_field
+    }
+  }
+`;
+
+const parser = new OnlineParser(source);
+let token;
+
+do {
+  token = parser.parseToken();
+} while (token.kind !== '<EOF>' && token.kind !== 'Invalid');
+```