Add a very basic ICS syntax parser

Summary:
Ref T10747. Extremely rough parser for ICS syntax, based on RFC5545 + RFC6868.

This doesn't understand any of the content in an ICS file or know anything about calendaring or events yet, it just applies all the weird linewrapping, escaping, typing and parsing rules.

I'll refine this to be less bad and eventually do useful things, but I got a basic test case running at least.

Test Plan:

• Added unit test.
• Ran unit test.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16520