added limited support for TypeScript-like field declarations

Summary:
A field declaration in a class body is recognized by
the nonterminal parameter followed by the nonterminal semicolon.

Test Plan:
The following code:

class A { x:string; foo(x:string) { this.x = x; } }

should now parse.

Reviewers: pad

CC: bryano

Differential Revision: https://phabricator.fb.com/D1041783
Merge branch 'fields'