Streaming API

Right now, it appears that using tinycss2 requires feeding the entire CSS file into the parser.

A streaming API (similar to html.parser) would allow applications to avoid buffering and the efficiency/security issues that it brings.

Start by reading tinycss2's current parser entry points and compare them with Python's html.parser streaming model. Define how CSS input can be consumed incrementally without buffering the entire file, and consider the efficiency and security concerns described in the issue. Done means applications can use a documented streaming API for CSS parsing.

Written by the indexing model from the issue text.